Sellers and purchasers of small- and medium-sized real estate are all smiles, and large-scale investors are glum, in the wake of Congress' last-minute solution to the imputed-rate controversy prior to adjournment.

Pressed hard by real estate groups, individual home sellers and investors, Congress agreed to delay through the first half of next year some of its 1984 tax reforms affecting seller financing. The hitch to the agreement, however, is that only transactions of $2 million or less are covered by the six-month freeze. All other properties -- whether apartment houses, commercial office buildings, farms or industrial real estate -- will be subject to the full effects of the 1984 tax reform act beginning Jan. 1.

Here's a quick rundown of what Congress did to real estate in its hectic rush to adjourn and prepare for the November elections:

If you're a purchaser or seller of any type of seller-financed property with a loan amount of $2 million or less and a below-market interest rate note, you can breathe easy until at least July 1, 1985.

The agreement passed by Congress rolls back the clock on all such transactions to the "status quo ante" that prevailed before the 1984 tax act was enacted last July.

In practical terms, if the house, land or other property in your deal comes with a seller-subsidized note carrying a rate of 9 percent or more, Uncle Sam won't impute a higher rate. If the sale carries a seller-financed rate of less than 9 percent, the Internal Revenue Service will define the true rate as 10 percent.

Let's say, for example, that you plan to sell a small investment property early in 1985 for $400,000. You are prepared to offer buyers a first mortgage or take-back note at 10 percent.

Under the original provisions of this summer's tax reform act, the IRS would have challenged your transaction next year, and penalized you for undercharging on interest. The IRS would have told you the $400,000 price tag was padded with deferred interest. The Treasury could say that the true economic rate charged to the buyer-borrower should have been at least 110 percent of what the U.S. Treasury itself pays when it has to borrow money for 10 years (just over 14 percent at today's rates).

Because you ostensibly undercharged your buyer, the 1984 tax law would have permitted the IRS to impute your interest income on the seller-financed note at 120 percent of comparable Treasury rates, more than 15 1/2 percent. (The fact that the seller actually was paying you only 10 percent interest and you did not inflate the price would have been irrelevant to the IRS under this system.)

The original 1984 law empowered the government to treat all assumptions or takeovers of existing, low-interest-rate debt on properties the same way. It exempted principal residences with sales prices of $250,000 or less and farms with sales prices of $1 million or less from the imputed interest rates, but included all second homes, vacation properties, and large numbers of small-scale investment properties and small firms.

The uproar over the tax law changes grew steadily louder from the summer onward, as sellers, buyers, accountants and other professionals began to grasp their effects. Congressmen were flooded with complaints from constituents. More than 300 members of the House of Representatives and half the members of the U.S. Senate indicated support for repeal or drastic overhaul of the new system.

Several major trade associations -- particularly terminate composition the National Association of Realtors -- mounted fierce lobbying campaigns aimed at redrafting the law or killing it before the end of Congress' final session.

As reported in this space last week, the Treasury Department, the principal architect of the 1984 reforms, came under such heavy political pressure that it agreed to roll back many of the imputed-interest provisions. Meeting over a 10-day period with representatives of the Realtors, the National Realty Committee, the National Multi-Housing Council, the National Apartment Association and other groups, Treasury negotiators worked out a lengthy series of revisions to the imputed-interest rules, all designed to lighten their impact on real estate transactions.

The Treasury's compromise plan, which key Senate and House leaders tried to push through Congress last week, would have covered larger-scale as well as smaller-scale real estate transactions.

The Senate passed the compromise plan overwhelmingly and sent it to the House for final approval. But House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) was in no mood to go along with what one aide called a "Republican rethinking" of the 1984 tax reform act. He convinced the House instead to offer a three-month postponement of the effective date of the imputed-interest provisions from Jan. 1. After late-night bartering with weary members of the Senate, both houses agreed to the six-month freeze, but only for transactions no larger than $2 million.

What's the outlook for further action in 1985? The odds are high that Congress will make significant, additional changes in the imputed-interest and so-called "original-issue-discount" provisions of the 1984 tax reform law in the first half of the year. The Treasury's detailed compromise plan hammered out with real estate industry groups is the most likely blueprint for future revisions.

In the meantime, the vast bulk of home sellers, vacation-home buyers and small-scale investors will get a reprieve from imputed-interest tax problems. Larger-scale investors, however, apparently will get hit with the tax reform headache right on schedule on New Year's Day.