The Supreme Court yesterday rejected claims by Zenith Radio Corp. that the federal government illegally forfeited up to $500 million in potential import fees in resolving a dispute over the sale of Japanese television sets in the United States.

Without comment, the court let stand a decision that the Carter administration acted properly in the 1980 settlement of charges that the Japanese had been "dumping" television sets in the United States by selling them at less than their market price in Japan.

In another business-related case, the court agreed to review a ruling partially shielding Cuba from lawsuits that followed the seizure of U.S. banks' assets during the 1960 Castro revolution.

Zenith, based in Glenview, Ill., said the government secretly agreed in April, 1980, to settle the dumping claims with 22 television importers for about $77 million. The importers included both large U.S. retailers and subsidiaries of the Japanese television makers.

The disputed televisions had been imported during the 1970s, when the U.S. television manufacturing industry was battered by Japanese imports. Of 11 American companies producing television sets in 1968, only General Electric Co., RCA Corp. and Zenith had not been liquidated or acquired by the end of the 1970s.

Zenith, which sued the government in May 1980 in an attempt to set aside the settlement, said the amount of anti-dumping duties the government could have sought was "in the vicinity of $600 million."

The government maintained it calculated the maximum amount of anti-dumping duties it could have collected was $138.7 million.

The U.S. Court of Customs and Patent Appeals last March ruled that the settlements were authorized, adding there was insufficient evidence of any governmental wrongdoing. The court then dismissed the suit.

Among those filing friend-of-the-court briefs urging the justices to hear the case and set down firmer anti-dumping guidelines was the Bethlehem Steel Corp.

In the case involving Cuba, the court said it will study a ruling that curbs the number of revolutionary government organizations that can be held liable for the nationalization of U.S. banks' branches. The case involved Citibank of New York and a 1961 lawsuit.

Cuba's Bancec, created to act as a foreign bank for Cuba's revolutionary government, sued Citibank for $193,280 allegedly owed to the Cuban commerce bank in a sugar sale. Citibank responded by saying it shouldn't have to pay the money because the Cuban revolution, of which it claimed Bancec was a part, had taken away Citibank branch assets that were worth more than the amount in dispute.