The controversial section of last summer's tax bill that lets corporations sell their excess tax breaks was promoted as a help to companies whose profits and tax liabilities were so low they would not otherwise benefit from the bill's changes in the tax code.

In fact, it turns out to be of enormous benefit to highly profitable firms as well--so much so that some will not only pay nothing to the Treasury in taxes but will actually make money from the tax code.

The buy-a-tax-break provision is turning out in this sense to be a negative income tax for these profitable companies as well as their weaker brethren.

The profitable companies use other sections of the tax code--depletion allowances, foreign tax credits, carried-over investment tax credits from past years--to reduce their tax liability to zero. Then they sell their tax breaks on new investments, converting credits and depreciation rights into cash.

Many companies are not making special announcements of such deals, but among those with significant income taking advantage of the tax provision are LTV Corp. (net 1980 earnings: $127.9 million), Occidental Petroleum Corp. ($710.8 million) and a host of railroads with solid earning records, including IC Industries Inc. and CSX Corp.

The arcane transactions grow out of a section of the tax bill allowing corporations to sell tax breaks through deals legally described as "leases." This provision was presented as a way to let unprofitable companies, particularly in steel, automobile manufacturing and paper, benefit from the tax-cut bill; unless they could sell their tax breaks they would not benefit, since they already owed no taxes.

The buy-a-break provision --which seems likely to cost far more than experts estimated when it was passed, and which some members of Congress now say they would like to retract--has provided a bonanza for profitable companies. To take an example:

Burlington Northern Inc., of Seattle, a railroad operating well in the black, not only received all the benefits of the basic business tax cut in the 1981 law but is a principal beneficiary of an amendment geared specifically to the railroad industry.

This amendment gave the industry the right to depreciate rail track estimated at $8 billion. For Burlington Northern alone, this meant a deduction in 1981 of more than $400 million on track valued at $1.1 billion.

The act "was indeed liberal in allowing railroads to depreciate their track structure," said R. C. Burton, vice president and treasurer. "The aggregate of all these benefits exceeds our ability to use them."

Last year Burlington Northern paid federal taxes of $29.5 million and, with profits looking better this year, would have expected to pay significantly more. With the tax bill, however, Burton noted: "Now we are not going to pay much of anything."

The firm had purchased $50 million worth of crossties and $69.9 million worth of freight cars and faced the prospect of being unable to use the tax credits and depreciation rights on these acquisitions.

The new tax law, however, also liberalized leasing provisions, allowing paper transactions in which the only things that change hands are tax credits and depreciation rights.

Burlington Northern took advantage of the leasing provision to sell its excess tax breaks to a group of companies, of which E. F. Hutton Group Inc., a financial services firm, was the only one publicly identified.

These companies paid Burlington Northern $36 million for the tax breaks on the crossties and freight cars. That price is only part of their tax-reducing worth. The E. F. Hutton group has bought itself more than $36 million in tax breaks; it and Burlington Northern both benefit.

In effect, it is the Treasury that pays Burlington Northern the $36 million; that sum is a government subsidy.

Although never described as a subsidy, the lease provision in the legislation, which was enacted with a minimum of public debate, was most often justified as a vehicle to spread benefits of the tax cut to beleaguered firms such as Ford Motor Co. and Chrysler Corp.

These companies, however, paid no federal taxes, and also had no net income, showing losses in 1980 of $1.5 billion and $1.7 billion respectively. Through leasing transactions, they will get what amount to federal subsidies--$26 million for Chrysler and between $100 million and $200 million for Ford.

To take the example of another profitable company, LTV Corp. had pretax income last year of $150.9 million. Facing a theoretical tax of $69.4 million, the Dallas conglomerate--with interests in steel, energy, aerospace, shipping and meat--paid federal taxes of $1.7 million after taking investment tax credits of $27.2 million, depreciation of $10 million, depletion allowances of $9.3 million and foreign tax credits of $5.3 million, among various writeoffs.

This compares with total federal tax payments by LTV of $18.1 million for the three years 1978 through 1980.

According to Robert Guyett, a tax specialist for LTV, the company has been having a very good year. But, with a backlog of $119 million in investment tax credits at the end of 1980 and net operating losses from past years of $218 million, LTV found that selling the credits and depreciation on 1981 investments became very attractive.

"Even though we . . . have been operating in the black, it makes economic sense to realize those benefits," Guyett said. "Through the new leasing rules, we have found the ability to accelerate the realization of these benefits."

To achieve this benefit from the federal government, LTV sold the tax breaks on $100 million worth of equipment used by a subsidiary, Jones and Laughlin Steel Inc., to an unidentified buyer for $33 million.

Similarly, Occidental Petroleum, which paid no federal income tax last year with a net income of $710.8 million, largely because of credits on foreign taxes, sold tax breaks on investments of $94.8 million for $20 million to $30 million to Marsh & McLennan Cos