The chairman of a key House appropriations subcommittee said yesterday he has obtained "agreement in principle" for President Reagan to commit his personal prestige to an attempt at reconciliation in El Salvador by appointing a high-level U.S. envoy to help arrange elections in which all parties to the conflict there may participate.

Rep. Clarence D. Long (D-Md.), chairman of the House Appropriations subcommittee on foreign operations, has been delaying consideration of the president's March 10 emergency request to transfer $60 million from other foreign aid accounts to increase military aid to El Salvador until Reagan and Secretary of State George P. Shultz meet this key condition. "There is still a little difference of opinion of what the man would do, but the idea of moving forward with this kind of thing has been accepted" by the administration, an aide to Long said.

"That's correct," Long said yesterday in an interview.

A senior State Department official said yesterday that the special envoy concept was "in the works" and that Thomas O. Enders, assistant secretary of state for inter-American affairs, and Alvin P. Drischler, deputy assistant secretary of state for congressional relations, had met with Long on Friday to discuss "what we would agree to."

"I think we need somebody whose stature is so great that once they appoint him they can't pull the rug out from under him," Long said. "What I have insisted upon is that we don't give them anything until they actually perform . . . on these conditions. It's up to them."

Long said he has told administration officials that he wants a presidential emissary of the stature of Middle East negotiator Philip C. Habib or President Carter's special envoy to that region, Sol Linowitz.

"Let them come up with somebody," said Long, "but it's very important to the public perception to get the peace process going, to have elections in which all people in the country are included and in which people can take part without being afraid of being killed."

Long said he will not call a meeting of the subcommittee to consider the president's emergency aid request "unless I get something in writing."

An aide to Long said State Department officials were sufficiently close to meeting Long's conditions last week that a subcommittee session on El Salvador was tentatively scheduled for this Wednesday or Thursday.

But Long said the process likely will take longer. If he reaches final agreement with the administration, Long said, he plans to fly to Central America and meet with Salvadoran President Alvaro Magana and Defense Minister Jose Guillermo Garcia "and tell them this is what the administration wants and they had better agree to this or they won't get the money."

The subcommittee's other conditions include the appointment of an independent American jurist to review FBI evidence in murder cases in El Salvador involving U.S. citizens and to recommend whether the cases have been properly pursued. Long also has asked Magana to allow spot inspections of the country's prisons by the International Red Cross and to declare a general amnesty for up to 800 political prisoners being held without charges.

The consensus of members of Long's subcommittee is to initially fund less than half of the $60 million requested by Reagan and to withhold approval for transferring the remaining portion as a means of ensuring progress on the political front, according to the aide to Long.

"It's one thing to appoint a negotiator, but three months from now, if they haven't moved forward on broad-based elections , then we wouldn't take any additional action," the aide said. "If there has been progress, then the members might want to provide the rest of it."

The aide said the administration is taking a gingerly approach to Long's key condition because State Department officials "are sensitive as to how far they can push El Salvador" in the electoral process before causing a revolt of dominant right-wing forces in the government.

"To be sure, this is not a discussion for power sharing, but to move toward an election process," the aide said.

The $60 million aid transfer is the first part of a $110 million military aid package for El Salvador that President Reagan announced in his March 10 address to the National Association of Manufacturers. He also is seeking $50 million in a supplemental appropriations package for the current fiscal year, which ends Sept. 30.

Reagan has asked for an additional $86 million in military assistance to El Salvador for fiscal 1984, which begins Oct. 1.

All of this would be supplemented by $200 million in economic assistance to El Salvador this fiscal year, and by $120 million in economic aid requested for fiscal 1984. State Department planners consider economic aid as part of "security assistance" because countries can use U.S. economic aid to pay other bills and to free remaining resources for military use.

The Senate Appropriations subcommittee on foreign operations has approved the full $60 million transfer request. But in a letter to Secretary of State Shultz, its members attached conditions that would limit the number of U.S. military advisers in El Salvador to 55 and require judicial reforms and attempts to negotiate with the leftist guerrillas without preconditions.

The Senate Foreign Relations Committee, which also asserted its authority to pass on budget transfer requests for foreign aid, has cut the president's $60 million request in half. It attached no conditions but urged the administration to seek "unconditional dialogue among the parties to the conflict in the hopes of achieving a political solution."

Meanwhile, the House Foreign Affairs subcommittee on western hemisphere affairs voted last Tuesday to cut the additional $50 million inmilitary aid for El Salvador for fiscal 1983,only to restore that amount in another vote Wednesday as development assistance grants to fight poverty.