The House Budget Committee completed work yesterday on a revised congressional budget for fiscal 1980 that calls for a deficit of $29.2 billion -- or $4.5 billion less than the panel had been considering on Monday.

Approval of the measure, which came on a vote of 16 to 9, could help strengthen the hand of Budget Committee leaders, who had been fearful that their earlier deficit figure of $33.8 billion might draw too much opposition.

However, the reductions were accomplished in large part through adjustments in revenue estimates that some analysts regarded as dubious -- including some $2 billion in "increased" receipts from a proposed speedup in collections of employers' withholding-tax payments.

The panel effectively sidestepped a proposal by the Senate Budget Committee to require Congress to make up for $4.3 billion in proposed cutbacks that various authorizing committees were supposed to make, but failed to enact.

The Senate is scheduled to begin floor consideration today of its own Budget Committee's recommendations, which call for a $28 billion deficit, based in large part on a demand that Congress cut elsewhere to offset its failure to act.

In any case, the two Budget panels seem headed for a conflict when the budget resolution goes to conference committee. Sources speculated last night the Senate would be unlikely to accede to the House Committee's actions.

Approval of the measure yesterday came after the House panel adopted intact a proposal by its chairman, Rep. Robert N. Giamo (D-Conn.) for a go-slow policy on President Carter's synthetic fuels program, providing for congressional review each year.

Carter has warned against such restrictions, contending it would only hamstring the synthetic fuels program. However, House leaders, including Rep. Thomas L. Ashley (D-Ohio), the House energy chief, insisted on the limitation.

The budget resolution the House panel approved yesterday eschew any provision for a tax cut, and rejects a call by conservatives for a sharp increase in defense spending. Both issues are expected to come up again on the floor.

Giaimo also is expected to oppose efforts by the Carter administration to push through a compromise proposal that would boost defence spending to a level 3 percent over this year's, after inflation, as Carter promised European leaders.

The White House announced yesterday the president formally approved that strategy to help fend off demands by Senate conservatives for an even bigger increase as the price for approving the strategic arms limitation treaty.

Republicans also are expected to move in both houses to push through their own proposal for a tax cut, which has become a centerpiece of their political strategy for this year and next.

The resolution approved by the House Budget panel yesterday calls for spending of $548.7 billion, up from $532 billion provided in the initial budget estimates of Congress passed last spring.

The proposed $29.2 billion deficit compares to a $23 billion red-ink figure in the earlier resolution. The increase stems primarily from more bearish inflation and economic projections.

Under the five-year-old congressional budget process, lawmakers set tentative spending and tax-revenue targets each spring and then review them in September to turn them into binding ceilings on outlays.

Along with the $1.5 billion in "increased" revenues that the panel approved yesterday, other so-called savings included $2 billion expected from a proposed speedup in payment by employers of withholding taxes.