In the special Washington language of the knowing wink and the unspoken promise, Rep. Charles B. Rangel (D-N.Y.) would like to think he has a multimillion-dollar understanding with President Carter.

For months, up until the evening of Aug. 17, the liberal congressman from Harlem had been opposed to the natural gas price increase bill that is a key part of Carter's energy plan.

Then, after meeting with Carter that evening at the White House, Rangel emerged as a supporter. His vote turned out to be one of the crucial last two that freed the gas compromise from a House-Senate conference.

The official explanation was - and is -that Rangel performed his sudden flip-flop on gas pricing "in the national interest" to help save the drooping dollar.

But there was, in that nuance of the wink and the nod, more to it than that. Rangel denies that he cut a deal with Carter. But he expects something in return for his vote.

What he wants, and what the White House knows that he wants, is special federal assistance for an extensive redevelopment project in distressed Harlem.

A centerpiece is a Third World trade center, with a new hotel and convention center, that would bring buyers, sellers, promoters and tourists together in central Harlem.

Rangel for months has been avidly promoting the trade-center concept, seeking Carter's support for it and pumping for federal money to get it started.

"For fear of being accused of dealing. I didn't discuss it with the president. I didn't think it appropriate to raise my case that evening, but I intend to," Rangel said this week.

"I hope the president in remembering my district, understands this as one of the hardest political decisions I've ever made, he continued, referring to his gas vote. "I would hope my continuing support of the president would be reflected in a more speedy reaction to the problems of my district."

Although Rangel denied repeatedly that specific promises has been made to him other sources - including one at the White House - indicated that the congressmen did receive executive-branch assurances of help for Harlem.

Rangel's frankness about his expectations seems bound to create more political problems for Carter in moving the natural gas compromise through Congress.

The first trouble bells rang this week in the Senate when Sen. James A. McClure (R-Idaho) disclosed that he gave Carter his vote on the gas compromise in return for a presidential promise to spend more money on liquid metal fast breeder reactor research.

The reaction among a number of senators was one of dismay that the president would imperil the compromise by trading for votes - in the wrong direction.

Two of the gas conferees Sens. Dale Bumpers (D-Ark.) and Mark O. Hatfield (R-Ore.), both of whom had aided Carter in his earlier battle against the breeder reactor, were so angry they said they would oppose the gas bill.

Another angry reaction came from Sen. Howard H. Baker Jr. (R-Tenn.), the minority leader who announced he would join a filibuster against the bill if necessary. Baker's opposition could be fatal to the measure.

The breeder reactor deal made its impression on Rangel, as well, who mentioned it in discussing the reasons for his change of heart on natural gas.

That is to say in the lingo of wink and mod, that he was aware the White House was interested in knowing more about Harlem's needs.

"Somebody get a nuclear reactor . . . others got water projects when the Panama Canal debate was going on," Rangel said. "The president has his constituency and I have mine."

Although he insisted that he made no deal for his vote Rangel conceded that he had broached the trade-center idea to Vice President Mondale during a chat before Rangel's meeting with Carter last week.

"He didn't know what the hell I was talking about," Rangel said.

But, the congressman continued, he had mentioned the idea to Carter as many as half-a-dozen times on other occasions. It had to be obvious, Rangel added, that Harlem very much wanted redevelopment help.

Through the Harlem Urban Development Corp., the community has pending at the Department of Commerce a request for some $330,000 to conduct a feasibility study and prepare architectural drawings for the trade center.

The corporation, of which Rangel is a director, along with other public officials, envisions the trade center as the linchpin in a major revitalization project in central Harlem.

Rangel said the grant request meant little or nothing "in terms of natural gas . . . I have more stuff pending at the Department of Housing and Urban Development for other related projects in Harlem and I wish Secretary [Patricia] Harris would call me about that."

Officials at Commerce and HUD adamantly insisted this week that they had received no signals or instructions from the White House to look more closely at the Harlem proposals.

Rangel, who claims credit for the trade-center idea, said the project could lead to way for "regenerating the whole economic corridor of the most strategic area in New York City."

The redevelopment plan includes a new state office building already built; a parking garage being built with federal aid; the trade center hotel and convention hall; a performing arts center, and new and renovated housing and shopping facilities.

Federal fund, through a number of agencies controlled by the White House, could be made available to the Harlem development corporation for these projects.