A Senate Appropriations subcommittee took a $1.9 billion bite out of the administration's hotly contested Labor-HEW money bill yesterday, arguing that President Carter had overestimated Medicaid, welfare and Suplemental Security Income costs for fiscal 1978.

The move immediately drew criticism from officials in the Carter administration. One described it as "an effort to trick the budget system."

"We call it low-balling," he said.

The subcommittee's action, Carter officials said, further complicated their efforts to cut back on House and Senate moves to raise spending for other programs in the Labor and Health, Education and Welfare bill.

Last week the President threatened to veto the money bill, which contains funds for the basic Democratic Party social programs, if it does not come in close to his own budget proposal.

The House Appropriations Committee has approved and sent to the floor a measure that contains $1.4 billion more for HEW than Carter wants.

And Warren G. Magnuson's Senate subcommittee last week added another $765 million to the House measure.

By chopping the three giant entitlement programs yesterday, however, the Magnuson subcommittee bill ends up close to the Labor-HEW spending figure that Carter originally proposed.

Yesterday, in a statement, Magnuson claimed, "This reduction will in no way affect the payments available to people eligible for benefits under these programs.

At issue between the Magnuson subcommittee and the administration is just now many people will ask for payments next year.

Magnuson said yesterday "the number of SSI (Supplemental Security Income) recipiants had declined by 7 per cent in the last 12-month reporting period" and "the size of families on welfare is declining." As a result, programs were cut a total of $800 million for next year.

Concerning Medicaid, the Magnuson subcommittee also forecast a drop next year in requests for funds. This one was put at a whopping $1 billion.

Carter officials don't agree with the Magnuson welfare decline estimates, but the Office of Management and Budget will not make its own projections until July - after Congress has completed voting on the Labor-HEW money bill.

Carter aides also argued yesterday that even if there is a decline, the entitlement funds should not be transferred to other Labor-HEW programs.

"That's like mixing apples and oranges," one aide said.

If the "oranges" are the entitlement programs, the "apples" in this case are what are termed "controllable" programs.These are funded at a level specified Annually in the appropriations bill - and not based on how many individuals apply for the money.

Past Congresses and administrations have played with the entitlement program estimates. In the current fiscal year, for example, the Ford administration projected Medicaid benefits at $10.9 billion, but Congress initially voted only $10.2 billion.

In April, however, a supplemental bill was passed with the additional $700 million.

That was not the end, however. Projections for the first seven months of fiscal 1977, according to congressional aides, show Medicaid payments from the Treasury were running $500 million below the first fiscal 1977 estimate.