The Reagan administration won a slim victory when the House of Representatives approved additional funding for the International Monetary Fund Wednesday night but the bill's opponents vow the fight is not over yet.

By a vote of 217 to 211, the House approved an $8.4 billion increase in the U.S. contribution to the international agency which has played a leading role in helping Third World countries with their debt problems. It must now go to conference with the Senate to settle differences between the Senate and House versions of the bill.

Because the vote was so close House opponents of the legislation may be able to prevent the conference from eliminating some of the House amendments that the administration and the Senate find objectionable, including one that prohibits the United States from voting for IMF loans to Communist countries and another that in effect prohibits the United States from voting for loans to South Africa.

The House approved the bill only after a heavy personal lobbying effort by President Reagan and Treasury Secretary Donald T. Regan.

"Given everything that's happened it's not going to be an easy conference," one congressional aide said. The South Africa amendment and the Communist country amendment are only two of the politically volatile parts of the House legislation that are not in the Senate bill, which passed handily in mid-June.

"We think when the conference report comes up in the fall another fight will come up," said an aide to Rep. Ronald Paul (R-Tex.), a vocal opponent of the legislation.