The Carter administration, struggling to keep its anti-inflation program afloat, made two key concessions in the critical Teamsters negotiations yesterday and announced a significant tightening of its five-month-old price guidelines.

At the same time, trucking industry and Teamsters negotiators reported substantial progress in narrowing their wage differences, raising new hopes that the two sides may be able to avert a strike by setting on a contract before Saturday night's bargaining deadline.

The developments came as President Carter, in his most serious expression of concern about the foundering guidelines program, told business leaders that if the Teamsters win an outsized wage boost "I don't think it will be possible to sustain" the 7 percent wage guideline.

Meanwhile, in the first official forecast that the economy may finally be cooling, chief Commerce Department economist Courtenay M. Slater said overall economic growth may have slowed to a 2 percent annual rate this quarter, down from a 6.9 percent pace in the last three months of 1978.

The administration's concessions in the Teamsters bargaining came in two parts:

First, the white House agreed not to count against the 7 percent wage guideline part of a 58-cent-an-hour cost-of-living increase due the union Sunday.

At the same time, the Council on Wage and Price Stability effectively softened its earlier opposition to proposed rate increases that the trucking industry said would be necessary to cover fully any wage boost the Teamsters win.

Although the council's new stance did not alter its previous objection to allowing the industry to raise prices enough to cover a wage boost that exceeds the 7 percent wage guideline, it did outline a partial escape hatch us- ing a rule that permits more flexibility for firms that raise prices yearly.

The two concessions were regarded as crucial by the industry and the Teamsters union. Sources said the White House move, which involves a bending of existing rules, effectively would "forgive" about 21 cents of the 58-cent wage boost. The concession to the truckers came in a letter to the Interstate Commerce Commission from Barry P. Bosworth, the council's director.

The tightening of the price guidelines essentially will limit the ability of companies to use a loophole in the earlier standards to avoid having to hold their 1979 price boosts to half a percentage-point below the average increase during 1976-77.

Under the standards issued last October, a firm either could follow the so-called "half-point deceleration" rule or else switch to an alternative limit under whch it promised to hold its profit margins steady. A profit margin is a measure of profit as a percentage of sales.

Bosworth said the tightening was ordered after an agency investigation showed "that the profit-margin standard is being abused." He said many firms were hastily shifting to the profit-margin test after it become clear they would breach the half-point rule.

Bosworth also said the council will set specific limits on how much beyond the basic guideline companies using the profit-margin standard may raise their prices. The agency also will ask those firms to file quarterly price and profit reports.

In addition, he said the council will seek detailed price reports from at least five industries in which he said officials have found evidence of excessive price-boosting-dairy products, pharmaceuticals, electric motors and generators, cement, and lead smelters.

Bosworth also made clear the council will use its subpoena power to obtain pricing information in cases where firms refuse to provide it voluntarily. He said recent inflation statistics have shown prices "are increasing at rates that cannot represent compliance."

The developments int he Teamsters talks appeared to signal a possible major breakthrough. Sources said yesterday union and industry bargainers were no more than 30 cents an hour apart on wages-down substantially from the $1.50 that divided them before a marathon session Wednesday.

Although chief industry negotiator J. Curtis Counts said Wednesday the two sides were in "complete disagreement" on money issues, the union-apparently anticipating the administration's concessions-sharply reduced its previous $2.35-an-hour wage demand.

At the same time, the industry raised its earlier 85-cent-an-hour offer.

The Teamsters also modified their proposal for a substantial cost-of-living increase, and the industry offered some improvements in benefits. But the two sides remained far apart on financing of benefits and other key issues, according to sources.

After more than eight hours of continuous bargaining yesterday, chief federal mediator Wayne L. Horvitz said the two sides were negotitating "sincerely and in good faith" but that "major differences still exist in economic and noneconomic areas."

Carter's meeting with the business leaders marked a stepup in presidential "jawboning" in behalf of the wage-price program. It was the first such session the president has had on the guidelines plan since the program began.

Although the session with Carter apparently went well, presidential anti-inflation chief Alfred E. Kahn reportedly exchanged words with U.S. Chamber of Commerce President Richard Lesher, saying, "For the life of me, I don't know why the Chamber is not interested in making this thing work."

Earlier, Kahn abruptly walked out during an appearance before the National Association of Manufacturers after his remarks drew criticism from NAM President R. Health Larry. The Associated Press reported that Kahn ahd sought to respond to Larry's remarks, but Larry "did not give him the microphone."

Carter's warning about the impact of a large Teamsters wage boost was echoed by Federal Reserve Board Chairman G. William Miller. Miller said in an interview if the Teamsters breach the guidelines "by any significant amount. . .I doubt that the program could survive."

In announcing the guideline changes yesterday, Bosworth blamed the excessive price boosts in the economy on a combination of higher-than-anticipated demand and efforts by some firms to beat out a feared imposition of mandatory price controls. Bosworth again denied such controls were being considered.