The D.C. government's employe retirement board, acting to avert a cash shortage, voted yesterday to take $6.8 million from its long-term investments account to make August and September pension payments for about 8,500 retired teachers, police and firefighters.

The action was necessary because pension advisers, for the second straight year, had underestimated the number of active retirement cases there would be and the amount of money that would have to be paid.

The board will lose about $85,000 a month in interest that the money would have earned if it remained invested, board officials said yesterday.

There is no direct dollar loss in benefits for recipients or taxpayers, but the use of long-term investments for short-term payments compounds the future financial problems of the board, according to city budget officials and municipal money-watchers.

Board chairman Frank Higgins told Congress in May that the board has projected a massive underfunded liability of about $8.8 billion through the year 2004 if Congress does not increase its $52 million annual payment to the fund.

Under a 1979 law, the city government is required to pay the yearly pension benefits based on the amount the pension board says it needs at the beginning of each fiscal year.

Although the law does not specifically require the city government to make up any shortfalls if the pension board underestimates its needs, some city and congressional officials believe that was the intent. "The current administration is putting off something on future taxpayers," one congressional source said yesterday.

Phillip M. Dearborn, a former financial adviser to Mayor Marion Barry, said however that he believes Congress meant for the pension board to make up any year-end shortfalls. "Now the pension board is unhappy and wants to change the argument" because it's underestimated how much it needed, he said.

"The basic long-term problem is so much bigger than the $6.8 million they are talking about" that yesterday's action will have little affect on long-term pension problems, Dearborn said.

Higgins, the board chairman, said yesterday that the board has been involved in a continuing dispute with Barry over whether the city government should make up the fund shortfall and has asked Congress for a clarification of its intention.

"It's the board's position that the city government is required to make up" the combined shortfall of $21.1 million, Higgins said.

Last year, the board was forced to withdraw $14.3 million from its long-term investments to cover a cash shortage when it underestimated the number of retirees.

In January, Barry gave the board a tentative commitment to put $4.3 million of those funds back in to the pension system in the 1983 fiscal year, which begins in October, but has refused to commit the city to paying any additional funds.