The Senate Appropriations Committee approved a $3.8 billion supplemental appropriations bill for the current fiscal year yesterday that would protect President Reagan's power to defer spending authorized by Congress.

The measure, which goes to the Senate floor, would more than double the new spending authority that the House approved last week in providing $1.7 billion in supplemental appropriations. As passed by the House, the bill would also have stripped Reagan of his authority to defer spending for policy purposes.

The White House has threatened to veto the House-passed version of the legislation. Senate Appropriations Committee Chairman Mark O. Hatfield (R-Ore.) said he was confident that the administration would endorse the bill as drafted by the committee.

In its report, the committee said it shared the House's "frustration" over the deferral issue, but said it recommended against adopting the House provision because of "the procedural complications it would create in the Senate."

The Senate bill contains numerous spending levels that differ from the House version and which will have to be reconciled in a House-Senate conference committee. The Senate measure, for example, includes $5.3 billion in new spending authority for the Commodity Credit Corp. that is not part of the House bill, as well as offsetting provisions to make up for this additional farm spending that are also not in the House legislation.

In another difference, the House authorized the immediate transfer of $702 million to beef up security at U.S. diplomatic posts round the world in the first step of a planned $4.4 billion security-enhancement program. The Senate committee, arguing that sufficient funds are available for this purpose, proposed delaying use of these funds until after the start of fiscal 1987 on Oct. 1.

During the committee meeting, Sen. William Proxmire (D-Wis.) failed in an attempt to delete a provision to transfer more than $80 million from various accounts to a series of defense-related research and construction projects at colleges and universities in the home states of several committee members. The proposed transfer includes $55.6 million for about 10 schools, plus $25 million that is specifically earmarked for Arizona State University.

Proxmire argued that the provision bypassed the normal competitive process for awarding such contracts, but he was defeated by 17 to 7.

The committee also rebuffed another effort by Proxmire to reduce the amount of money senators can be paid from outside speaking and writing engagements. The committee, by 13 to 8, tabled his motion to reduce this from an amount equal to 40 percent of a senator's salary to 30 percent.

Budget Committee Chairman Pete V. Domenici (R-N.M.) said the supplemental appropriations measure would not affect the deficit this year because most of it is new budget authority that will not be used until later. He said the measure as drafted by the committee would result in an extra $785 million in spending in fiscal 1987.

Domenici said his latest estimate for the fiscal 1986 deficit was $210 billion to $215 billion, an increase from his earlier estimate of slightly more than $200 billion.