A secret 1973 message from Richard M. Nixon, promising North Vietnam billions in economic aid was made public yesterday.

Rep. Lester L. Wolff (D.N.Y.) also released an explanatory letter to him from Nixon in which the former President said his promise of aid was not part of a "price" to obtain a Vietnam peace agreement.

Nixon's letter was the first confirmation by an member of his administration of the 1973 aid promise.

His message to Vietnamese Premier Pham Van Dong containing the promise of a "range of $2.25 billion" of reconstruction aid and $1 billion to $1.5 billion of food and commodity aid was made public yesterday by the State Department, which obtained it from Nixon's papers.

Wolff said that former Secretaries of State William P. Rogers and Henry A. Kissinger had misled Congress by not informing it of Nixon's aid pledge.

He also said he will seek a copy of Pham Van Dong's response, which reportedly confirms the aid understanding. Presumably, that response, like Nixon's promise, was not kept in State Department files, but among Nixon's presidential papers.

As recently as last July 21, Under Secretary of State Philip C. habib told a House committee that Congress had been provided with all the documents relating to Vietnam peace agreement and that the United States never pledged a specific dollar figure.

Nixon's message to North Vietnam is dated Feb. 1, 1973, four days after the Paris agreements were signed. It appears to be a part of the agreement, Wolff said, because it seems unlikely that specific amounts of aid would have been determined in only a few days.

A recurring element, although not always a secret one, in Kissinger's diplomacy was U.S. payments of aid, most notably in his Mideast diplomacy and in his unsuccessful effort to bring majority rule to Rhodesia through peaceful means.

Wolff said he will call Kissinger, Rogers and Habib before the House International Relations subcommittee on Asian and Pacific affairs, which he chairs, to explain the secrecy surrounding the promise of aid to Hanoi.

North Vietnam still refers to the promise as binding, but Wolff and a panel fo congressman appearing at a press conference with him made clear they were making the text of the promise public in order to bury it.

They all agreed with Nixon who in the explanatroy letter to Wolff wrote: "There is no commitment of any kind, moral or legal, to provide aid to the Hanoi government," because of Hanoi's violations of the peace agreement in its ultimately successful military campaign to conquer South Vietnam.

President Carter has said his administration will not make aid payments as a precondition to normalization of relations with Vietnam. Congress has passed a law forbidding any aid to Vietnam.

Wolff noted Hanoi's continuing references to the Nixon pledge and said, "We have to put that to rest." By making the document public, he intended not to give Hanoi hope of aid but to encourage it to negotiate seriously with the Carter administration and realize that any Nixon promises have been rendered void.

In the same vein, Nixon wrote Wolff, "I can think of no action which would be less justified or more immoral than to provide any aid whatever to the Hanoi government, in view of their flagrant voilations of the peace accords."

Nixon was replying to Wolff's Feb. 22 request for a copy of the aid pledge.

The former president did not respond unitl last week when former National Security Council Chairman Brent Scowcroft telephoned Wolff on Nixon's behalf to say that a letter of explanation was being prepared in San Clemente.

By then, Nixon was aware that Wolff had seen a copy of the promise and that the State Department was responding to the congressman's request to declassify the document and make it public.

Nixon begins by citing executive privilege, which he says protects him from having to produce the document, but then provides his recollection of events voluntarily.

His letter never mentions the dollar amounts promised to Hanoi, but he says he briefed a bipartisan congressional leadership group with Kissinger and Rogers Jan. 23 and 24, 1973. "The subject of aid was discussed at some length and there was, as I recall, virtually unanimous support for the proposals provided it would serve the purpose of helping assure adherence to the Peace Agreement."

Nixon says that Nanoi was never under any illusion that it could receive aid without the approval of Congress and without observing the peace terms.

The former President's message to Vietnam contains a clear statement that aid would need congressional approval. That statement, although only one paragraph long, is on a separate page from the main message.

Wolff asked rhetorically at his press conference whether the pledge to seek Congress' approval was on a separate page so that it could be easily detached or in order to give it special emphasis.

A second separate page at the end of the agreement spells out the $1 billion to $1.5 billion "depending on food and other commodity needs of the Democratic Republic of Vietnam" that could b available.

The key paragraph of the message says:

"Preliminary United States studies indicated that the appropriate programs for the United States contribution to postwar reconstruction will fall in the range of $3.25 billion of grant aid over five years. Other forms of aid will be agreed upon between the two parties.

"This estimate is subject to revision and to detailed discussion between the government of the United States and the government of the Democratic Republic of Vietnam."

Nixon said the U.S. reconstruction aid was to be "without any political conditions."