The U.S. merchandise trade deficit slipped to its lowest level of the year last month, $9.9 billion, which some administration officials saw as a potentially helpful sign for President Reagan's push against protectionist pressures in Congress.

The August trade figures, announced yesterday by the Commerce Department, showed the deficit was $1.7 billion less than the average for the first seven months of the year, and $41 million less than the July figure. The monthly deficit was 26 percent below the year's high, the $13.4 billion imbalance recorded last June.

The dollar was modestly higher in European markets yesterday, buoyed by the lower than expected trade deficit. But the dollar fell sharply against the Japanese yen, the principal focus of coordinated action by the United States and its major trading powers to bring the dollar down.

Economists were divided on the reason for the dip in the trade figures, with one, Michael Evans of Evans Econometrics, hailing it as "no fluke.

"It's the second month in a row we've had a real break with the past." He said two-thirds of the improved trade performance was due to declines in imports caused by the early-year drop in the value of the dollar.

Commerce Secretary Malcolm Baldrige, however, took a less optimistic view in testimony yesterday before Congress' Joint Economic Committee. "These figures are simply one more indication of a trend that should concern us all," he said. "They reflect the continued erosion of our trading position worldwide. They confirm that the United States continues to buy more than it sells abroad."

Stephen Cooney of the National Association of Manufacturers said the main factor in the lower trade deficit is the slowdown in American economic growth, which brought a lessened demand for imports.

"It's too early to say we are seeing a turnabout in trade," declared Larry Chimerine of Chase Econometrics.

Nonetheless, the decline in the trade deficit buoyed Reagan administration officials who have been fighting what appears to be a losing battle to stem a flood of trade bills. Many of these bills are protectionist, coming from a Congress which has become increasingly concerned about trade as a potent political issue related to job losses.

Administration officials said the improved trade figures could give congressmen an out if they are waivering in their support of protectionist bills. The new figures could also help President Reagan gain enough votes to sustain an expected veto of a measure to slash textile imports, which is likely to pass Congress within a month.

The deficit for the first eight months of the year totaled $91.2 billion -- 8.6 percent greater than for the same period in 1984 -- and government officials still believe the 1985 deficit will come close to $150 billion.

But Baldrige told the congressional committee that the deficit could decline to $130 billion from $135 billion next year as a result of President Reagan's new, aggressive trade program. The new presidential initiatives include the coordinated action with major industrialized nations to force down the value of the dollar; a $300 million war chest to counter subsidized export financing, and a more aggressive attack on unfair trade practices that hurt U.S. exports and increase imports.

The Commerce Department also revealed yesterday that a recompilation of trade figures for last year showed that the trade deficit was $127.6 billion -- $4.3 billion higher than had been originally reported. Commerce officials said the change was due to late information supplied by the Customs Service.

The August figures showed that imports dropped 2.1 percent from July levels, largely because of declines both in the amount of petroleum shipments and in the price of oil. There was a 4.6 percent decline in petroleum imports, and the price dropped an average of 20 cents a barrel to $26.96.

With winter approaching and oil stocks drawn down, however, economists expect petroleum imports to grow in the coming months. It is unclear how the price will move.