The District's urban renewal agency voted yesterday to require Banneker Associates to buy the prized Portal property within six months--a dramatic policy change that gives the developers one-fourth the time usually allowed for purchasing city redevelopment property.

By imposing the deadline, the first of its kind, the Redevelopment Land Agency is attempting to generate more money for the city treasury through quicker land sales and to force developers to build more promptly.

The RLA board warned prospective developers of the early purchase requirement two months ago and picked the Banneker team from among four competitors, in part because of that group's strong assurances that they could meet the deadline.

Yesterday, as RLA board members maneuvered to keep the deadline as tight as possible, Banneker attorney and partner S. Lee Narrow tried to weaken it by tying the land purchase to the developers' obtaining the necessary approvals from other federal or District agencies that must pass on the $355 million office building and hotel project.

He wanted that link to ensure that his group will be able to build on the land when they buy it, to avoid the expense of paying high interest rates on idle land.

The board rejected his proposal.

The Banneker group now has two days to sign the development agreement, which includes the 180-day deadline, and to give the city a $50,000 deposit for the site at the foot of the 14th Street bridge.

Narrow said the board's refusal to link the purchase with approval of permits was unexpected. He demurred when asked after the meeting whether his group, which includes well-known developers Theodore N. Lerner and Melvin Lenkin, would sign the agreement. But he added, "We don't want to stop the process at this point."

The board said it would consider an extension if the developers proved that they had not caused any delays with other agencies. The board also agreed to extend the deadline if the RLA itself caused delays.

The developers also wanted the right to give special instructions for the RLA appraisal that will help determine the price the developers will pay for the Portal site. The board said no.

RLA chairman Nira Long said the request would abridge RLA's rights as a seller to determine a price for its land. After the meeting, Narrow refused to explain why the developers wanted the special instructions.