The Federal Home Loan Mortgage Corp. (Freddie Mac), under pressure from Congress, has postponed its controversial plan to end homebuyers' assumptions of low-interest mortgages it has bought.

The postponement gives Congress time to consider legislation on the assumption issue. It also means that homeowners trying to sell their homes right now will get more time to take advantage of assumptions if Freddie Mac has bought their mortgages.

With mortgage interest rates around 16 1/2 percent, the ability to assume a homeowner's old, lower-interest mortgage at the old rate can be worth thousands of dollars to a buyer and seller.. But lenders want to be able to call in those low-interest loans when a home is sold. They say that any restriction on their right to do so costs them large sums of money at a time when they are facing severe financial problems.

Until recently, Freddie Mac's policy was that all mortgages it bought were fully assumable at their original interest rates, but on July 2, the corporation announced that as of Aug. 2 all those mortgages would be nonassumable.

The announcement caught Congress and members of the housing industry by surprise.

Sen. Jake Garn (R-Utah), chairman of the Senate Banking Committee, has been working on legislation since last year to resolve the issue of mortgage assumptions and told Freddie Mac he would sponsor legislation to postpone the new policy until Congress could consider the issue.

Yesterday, the corporation decided it might as well delay the change itself. Its new policy will not go into effect until next Jan. 1 or until Congress acts, whichever comes first, Freddie Mac officials said.

One possible compromise is for rates on its loans to be "blended" when a home is sold, the rate established somewhere between the old rate and the market rate when the sale takes place.

Jack Carlson, chief economist at the National Association of Realtors, said Freddie Mac's announcement will help "take away some of the chill" in home sales for the rest of this year.