President Reagan has agreed ''in principle'' to increase, if it proves necessary, the number of American peacekeeping troops stationed in Lebanon, according to a Lebanese special envoy.
The special envoy has asked for up to 6,000 more marines, but affirms that Lebanon's Army can keep peace. Meanwhile, diplomats say Syria may withdraw its forces - if the price is right.
The envoy, Sa'eb Salaam, a former Lebanese prime minister, said it may be necessary to increase the multinational force now in Lebanon from its present strength of 4,000 to between 7,000 and 10,000.
Mr. Salaam, who presented President Reagan with a message from Lebanese President Amin Gemayel on Thursday, said a beefed-up multinational force would be needed ''to make absolutely sure that the Lebanese Army can replace th deficit or fund jobs programs.
The Lebanese apparently believe that if the multinational force, which includes 1,200 US marines, can expand its role, it will be more difficult for Israel to refuse to withdraw from Lebanon. The Israelis, howeverit isn't red!
While finishing President Reagan's proposed '84 budget, administration officials faced a tough decision: What color should the budget book's cover be? The problem ended up on the desk of an assistant to Office of Management and Budget director David Stockman. After poring over a wide variety of shades and styles, says a high OMB source, the assistant decided, ''I like red.'' Suddenly, visions of the record amounts of red ink predicted by the document sprang into his head., object that neither the Lebanese Army nor a multinational contingent would be sufficient to secure southern Lebanon from infiltration by the Palestine Liberation Organization (PLO).
''The Israelis go on disputing the ability of the Lebanese Army to take over, '' said Salaam in an interview. ''Yet we have assurances not only from our commanders but also from the US generals themselves on this point. . . .''
An administration official confirmed that Mr. Reagan agreed in principle to increase, if need be, the size of the American marine force in Lebanon. But he added that this would have to be done in coordination with the other allied nations involved. The official also said that there was a feeling of urgency in some circles in Washington because the Lebanon talks had delayed the start of negotiations on the Palestinian issue. He noted that while the current talks between Israel and Lebanon ''dragged on,'' the Israelis continued to build new settlements on the occupied West Bank of the Jordan River.
Officials see a need to swiftly conclude the Lebanon talks for two other reasons as well:
* American intelligence indicates that PLO fighters are gradually returning to Beirut from other parts of Lebanon. They are entering the Lebanese capital in civilian guise, and at this point may number in the hundreds, an official said. American officials are convinced, however, that if the Israelis agree to withdraw from Lebanon, the Syrian and PLO forces will withdraw as well.
* Harassment of US marines by Israeli troops has apparently increased. Defense Department officials have confirmed a report carried by the Knight-Ridder News Service in which the commandant of the Marine Corps charges that Israeli troops have deliberately and repeatedly threatened the lives of marines and US Army officers.
The news service said Marine Gen. Robert H. Barrow complained to Secretary of Defense Caspar W. Weinberger that the Americans were being threatened as part of an orchestrated campaign ''executed for obtuse Israeli political purposes.''
The report added there is speculation in Israel that some Israeli military leaders would like to make life so unpleasant for US forces in Lebanon that the American public would object to a stronger role there.