HEAL THYSELF, the United States told the United Nations several years back: reform politically and managerially and we'll pay up. This was the basis on which the Reagan administration openly encouraged a Congress that needed little encouragement to renege on solemn treaty-mandated financial commitments to the world organization. Even those people troubled by the harshness of the medicine found it hard to make a case that there was any other way to move the U.N. back toward fairness and efficiency.

What happened then is not so well known. The United States pretty much won. It got visible progress -- not full, but substantial -- on budgetary and structural reform. The jackal-pack quality of much General Assembly debate, on Israel and the like, eased a bit. A couple of the world's rawest regional disputes, in Afghanistan and the Persian Gulf, were taken on by a responsible secretary general. Meanwhile, changes in the Kremlin produced a leader who was ready to start using the U.N. relatively less for propaganda and more for serious diplomacy. All this happened as Ronald Reagan, who has visited the place more times (six) than any other American president, confounded assertions of his unregenerate hostility to it and began to make greater use of the U.N.'s services himself.

In short, the stage was set for resumption of full American payments. But the American government has remained in default. The Washington-based approach to the budget taken by the president and Congress has simply failed to produce the funds Mr. Reagan promised to pay in New York upon the U.N.'s delivery of reform. The Russians are paying up, but the Americans are not. The result is that what the secretary general calls ''minimum austerity operating expenses'' are not being covered, the future of reform is clouded, and the advantages gained at heavy cost for American interests threaten to be dissipated.

The two houses of Congress will shortly go to conference on the matter, both having already voted sums well under what the administration sought. It should be regarded as absolutely unthinkable for the United States to continue as a deadbeat at the U.N.