Bowing to budgtary and political difficulties, the United States is preparing to back down on its promise to substantially increase its aid to developing countries, according to official sources.

In giving tentative approval to recent decisions by the Office of Management and Budget, President Carter informed aides that he is not abandoning the U.S. promise but is postponing it due to economic circumstances, the sources said.

Carter's decision not to provide the promised increase in assistance to the developing world contrasts with his reported determination to make good on his promise of a 3 percent increase in U.S. military spending. This and other aspects of the recent budgetary decision-making have caused dismay among some officials who know of them, and may bring a sharp reaction from Third World countries.

Administration officials said Carter is also taking a strong stand against any major new aid package to back up the anticipated Egyptian-Israeli peace treaty.

Earlier, Vice President Mondale and other senior administration aides expected that a large increase in aid to Israel and Egypt would be an important piece of legislation in the new Congress. Carter surprised them by saying flatly that he is not committed to a major program for this purpose, despite widespread belief to the contrary.

Carter's budget request to Congress mext month will include a bilateral foreign aid figure nearly the same as last year's $4.5 billion request, according to administration sources. The State Department and the Agency for International Development (AID) are expected to mount a routine appeal of this decision, but there is believed to be little likelihood it would succeed.

The budget request will also call for about $1 billion in U.S. capital for international financial institutions such as the World Bank. This is money to makes good on past pledges that have not yet been matched by appropriations.

During his campaign for the presidency, Carter promised to do more for the world's poor nations. As president in April 1977, he approved a high-level interagency recommendation that the United States substantially increase the flow of development assistance to the Third World.

The decision was made public in May 1977 by Secretary of State Cyrus R. Vance in a speech in Paris to the Conference on International Economic Cooperation, the leading forumfor North-South dialogue, which included 16 industrialized countries and 19 developing ones.

Vance announced then that "President Carter will seek from Congress a substantial increase in the volume of our bilateral and multilateral aid programs over the coming five years."

The pledge was later refined by Carter and OMB to a goal of $10 billion in total U.S. economic assistance by fiscal 1982.

Taking their cue from Carter's private mesage to aides, U.S. officials are preparing to argue that the failure to increase the foreign aid budget is a temporary expedient that does not reflect a permanent change in policy. They will argue that once the United States is able to get its economic house in order, developing countries will benefit from growing trade as well as aid.

AID Administrator John J. Gilligan, who declined a request to be interviewed about the budget decisions, is reported to have expressed unhappiness at the White House.

A White House officials said it is simply "a fact of life" that a substantial increase in foreign aid is impossible at a time when the administration is calling for cuts in more popular domestic programs. Given the political climate of the new Congress, the foreign aid funds are believed to face a particularly serious test regardless of the level of the budget request.