Because of words dropped from an article on bank loans to Mexico in Sunday's Business & Financial section, the impression was conveyed wrongly that the International Monetary Fund required the banks to levy extra interest and fees. It should have said that the extra charges were the end result of the IMF position that if the banks did not help Mexico the IMF would not make emergency "stand-by" loans.

Major banks here and abroad, by charging premium interest rates and stiff special fees, have extracted a bonus of more than $800 million from Mexico as the price of maintaining their lending relationships with that hard-pressed debtor country.

Similar fees and extra interest are being charged by the banks to other countries as their debts are rolled over in cooperation with emergency programs financed by the IMF.

Criticism of this practice is mounting among those who fear that it will come back to haunt the banks and will adversely affect not just the economies of the borrowing countries, but of the richer nations where the banks are domiciled as well.

On Thursday, Mexico--whose export earnings are down sharply--appealed to the banks for permission to delay until Aug. 17 any further repayment of principal on its loans. Last November, Mexico was given a delay that expires March 23.

The net result is to increase the annual burden that the poor countries face in servicing debt. In a worst-case scenario, says Rep. Charles E. Schumer (D-N.Y.), the bankers' terms, combined with "a business-as-usual approach by the Reagan administration, would leave the banking system constantly on the brink of disaster and subject to endless threats of default."

Schumer and others contend that, if Congress agrees to the administration request for $8.4 billion in enlarged IMF quotas and for participation in a separate emergency fund, the IMF should be required to work out a plan to convert much of the existing short-term Third World debt into long-term debt at lower interest rates.

Concern over the "extras" now being charged all LDCs (less developed countries) by the banks is apparent in some parts of the administration. In a speech March 7, Economic Council Chairman Martin S. Feldstein warned that the banks' high charges to poor countries may prove to be self-defeating. "By increasing the total interest bill of the borrowing nations, a higher risk premium increases the risk of nonrepayment," Feldstein said.

Banks do not publicly report their fees and interest rates. But in the case of Mexico, according to bank expert Karen Lissakers of the Carnegie Endowment for Peace, the banks had charged a rescheduling fee of 1 percent, plus an interest premium ranging between 1 7/8 and 2 3/8 over standard international lending rates, as the price of converting $20 billion in short-term loans to eight-year loans. That's an average of 3-plus percentage points, or $600 million in profits, added onto the old loans.

"I think it is just outrageous," Lissakers said. "The banks are saying, 'Help! Help! the patient is dying. The International Monetary Fund, the Bank for International Settlements, the U.S. government have to rush to the rescue.' And at the same time they're tapping the last drop of blood out of the body."

In addition to rescheduling the $20 billion, the banks agreed under pressure from the IMF to lend Mexico $5 billion in new money. For that, they are getting 2 1/4 points over the London Interbank Offer Rate (LIBOR), plus a fee of 1 3/4 points. The extra 4 points on $5 billion comes to $200 million. Total extras to be paid by Mexico: $800 million.

Feldstein noted that the same banks just a few years ago were lending to poor nations for less than the U.S. prime rate and only a fraction over LIBOR.

To some, the banks' current operations represent profiteering at the expense of creditors already driven to the wall. The response by the banks is that the greater risks they are now taking demand higher interest rates and that, since they underpriced loans to Mexico and other Third World nations in the first place, they must now recoup.

In addition, the large banks say that by insisting on a sweeter package in conjunction with rescheduling, they have persuaded regional banks to "stay in the lending picture."

But, says Lissakers, "a time of emergency is hardly the time to make up for the underpricing of the past. All they the banks are doing is putting new money in with one hand and taking it out with the other. What good does that do?"

As for the regional banks, the probability is that they would have "stayed in" for a smaller bonus; in effect, they had no other option.

Schumer agrees with this analysis. He complains that, instead of being required to pay a price for their past miscalculations, the banks are now profiting from a rescheduling process that, in effect, is being underwritten by the IMF.

" The IMF is saying to the banks: 'If you'll continue to put more short term money into these Third World countries so they can repay their previous debt, then we will basically insure, not only that you don't have to take any loss, but that you can make very, very sweet profits on these loans,' " Schumer says.

In an interview in his Cannon Building office, Schumer, a member of the House Banking Committee, said that it would be counterproductive if Congress approves Reagan's request for $8.4 billion in expanded IMF resources and does nothing to consolidate and lengthen the Third World debt.

"If it were just one country that was insolvent, and not a whole world in recession, the Reagan plan might work," he said. "But under present circumstances, it is unlikely to succeed."

The extra interest and fees now being levied by the banks, the critics charge, amount to an ultimatum to the banks from the IMF not only to keep and reschedule old loans made to the poor countries, but also to put fresh money in. They note that IMF Managing Director Jacques de Larosiere has made clear that the IMF would not agree to make emergency "stand-by" loans.

After intense negotiations over the past several months, about 500 commercial banks, including 170 big and little ones in the United States, agreed to lend Mexico $5 billion over and above existing loan totals. The IMF then agreed to go ahead with its own emergency $4 billion loan. This provides the rationale by which de Larosiere claims that the IMF is not bailing out the banks, but "bailing them in."

In the case of Brazil, the IMF put up $4.9 billion, when the bankers agreed to new commercial loans of $4.4 billion as well as substantial trade credits.

Without the infusion of the IMF and new bank money, Mexico almost surely would have defaulted on some of its estimated $80 billion debt, precipitating an even deeper world economic crisis. Brazil, with another $80 billion to pay off, might have been almost as close to the edge.

While virtually all of the larger banks and many of the smaller ones yielded to de Larosiere's pressure, they set their own terms in interest and fees, sources say. Originally, according to reliable reports, they were asking even more. But Lissakers thinks that the IMF and the Federal Reserve Bank together could have and should have exercised enough leverage over the banks to have brought down the current fees and "spreads" over international interest rates.

Beyond that, a New York investment banker in close touch with the international lending scene is bitterly critical of the way the banks "book," or report these "fees."

"It's specious as hell, and very bad financial management if they take those fees and report them as income. If anything, those fees--paid up front, and far in excess of actual renegotiation costs--should be set aside as reserves against losses," he said.

Feldstein put his warning note in a very cautious framework. He is not antagonistic to the banks. In an interview, he stressed his belief that, given a choice, the commercial banks now stuck in the LDC lending process "would rather be doing something else with their money" than extracting premiums and fees from Mexico and other poor nations.

"That doesn't detract from the fact that these rates also make it that much more difficult for the country to repay and, therefore, raise the riskiness of the whole operation," Feldstein said. "So you have the irony that the banks, quite understandably, want higher rates to compensate them for greater risk. But the higher the rates they charge, the more risk there is." Nonetheless, Feldstein defends the austerity imposed on the debtors because "there is no alternative."

Schumer and others argue there is another way of approaching the problem. The Brooklyn Democrat starts from the premise that the banks initially shoveled out the money without thinking much about the consequences and that it would be compounding the error for the Reagan administration now to join forces with them and with the IMF to protect the banks' investments in a way that will squeeze economic growth here and abroad.

The objective of the stretch-out proposals is to reduce LDC annual repayments of interest and principal, which would allow them to increase imports from the United States and other countries. Variations on this theme have been proposed by Sen. Bill Bradley (D-N.J.); New York financier Felix Rohatyn; and Princeton economist Peter B. Kenen.

Despite opposition by Fed Chairman Paul A. Volcker, more attention is being given to the possibility of setting limits on the amount of loans any bank can make in one country. Legislation to this effect has been introduced in the Senate by William Proxmire (D-Wis.) and John Heinz (R-Pa.)

What worries Schumer most--and what runs as a theme through the Feldstein speech--is that the austerity enforced on the debtor nations will cause them to slash their imports, which not only means belt-tightening in the poor countries, but will be costly to sales, profits and jobs in the United States