For anybody considering a contribution for the rebuilding of the Filene Center at Wolf Trap, there is encouraging news: President Reagan, who early on pledged help, is making good on the commitment. The administration will provide half the money needed for the effort and lend the rest while the campaign for private donations proceeds.

This assurance of money up front can speed construction. Catherine Filene Shouse, whose gifts made Wolf Trap Farm Park possible in the first place, says the cash might even get the job done by June of next year. That may be too optimistic, but with congressional approval of the administration plan, the whole project should move faster. The loan also gives would-be contributing businesses a clearer incentive to respond right away.

Details of the loan are still to be worked out. But with a reasonable rate of interest and a reasonable period for private contributors to pay back (say, six years), the Wolf Trap Foundation can pinpoint the amount it needs to raise, outline a timetable and seek the necessary support. Too short a term at too high an interest rate could dash all hopes of finishing the project quickly.

While President Reagan's 50-50 plan is sensible, it should be made clear around the country that this is not some government subsidy of a private highbrow project. This is a federal building on federal land, owned and operated by the government, in a park that is enjoyed by people from all over the country. Thanks to the generosity of many people and businesses, fund-raising has already begun.

Much will be needed--perhaps $10 million from private donors. They may send checks to the Wolf Trap Foundation, c/o The Postmaster General, Washington, D.C. 20260. Congress can help, too, by expediting any legislation necessary to authorize and carry out President Reagan's proposal.