The Carter administration said yesterday its new treaty provision allowing foreign firms to bid on more government procurement contracts here will affect less than $300 million in orders previously earmarked for minority and small businesses -- not $9 billion worth.

The revised figures were provided by Robert S. Strauss, Carter's special trade negotiator, after reports of the administration's action drew a storm of protest. The $9 billion figure was compiled hastily by another agency after Strauss' office said Tuesday it did not have any estimates.

Strauss said the $300 million figure was more accurate because the $9 billion estimate did not account for special restrictions in the treaty provision which guarantee existing preferences for minority-owned and small business firms here on textiles, shoes, stainless steel and other products.

However, both congressional and industry spokesmen immediately disputed the lower figure. And Arthur Johnson, an official of the National Association of Black Manufacturers, complained the move still "reverses" Carter's commitment to minority-enterprise efforts.

The provision is part of a package in the newly negotiated multilateral trade agreement that policymakers say was designed to prod European nations and Japan to open up more of their government procurement business to American exporters.

Strauss said yesterday the changes will enable U.S. exporting firms to bid on some $25 billion worth of foreign government contracts in exchange for opening up $10 billion worth of U.S. government contracts to overseas manufacturers. Most of the U.S. orders will go to big firms.

Strauss argued yesterday the trade was a fair one because until now U.S. firms have been barred entirely from bidding on government contracts in Japan and most European countries.