THE ACCLAIM Nelson Mandela won in Washington and the evident additional support he has generated for the African National Congress may sharpen the question of the terms on which sanctions against South Africa should be eased. Mr. Mandela launched a powerful plea to Congress, ''to cede the prerogative to the people of South Africa'' of determining when to lift sanctions. This puts him in collision with the anti-apartheid law of 1986, which places that prerogative firmly in American hands and specifies terms for terminating sanctions that are substantially milder than those he has in mind.

When would the ANC find progress toward a just society to be sufficiently ''irreversible,'' as Mr. Mandela has repeatedly put it, to justify the end of sanctions? To Congress, he outlined a five-stage progression: 1) removing obstacles to negotiations, 2) negotiating a mechanism to draw up a new constitution, 3) forming that constitution-making institution, 4) writing a constitution and 5) holding elections. Stage One is where events are at the moment. Stage Four -- and not just in the writing of a constitution but perhaps in the final agreeing on it -- is where Mr. Mandela says he expects the condition of ''irreversibility'' to kick in.

The Comprehensive Anti-Apartheid Act of 1986, meanwhile, provides for ending sanctions simply if the government frees political prisoners, repeals the state of emergency, unbans all parties, repeals the Group Areas and Population Registration Acts and ''agrees to enter into good faith negotiations.'' When the law was written, these standards were seen as ambitious and demanding. But events have largely overtaken the law. Those of its terms that have not already been met may come within Pretoria's reach during the next six months or so. President Bush told Mr. Mandela the United States would proceed in this matter according to ''the conditions laid down in our law.''

This is the right course. The familiar argument may continue about who is helped and who is hurt by sanctions. Supporters of the ANC position tend to find a moral aspect to the decision. But there is good reason for all parties in South Africa to understand that the United States is faithful to its word.