The development firm that initially agreed to pay the city $45 million in cash for the coveted Portal Site near the Southwest Washington waterfront is now seeking to cut the price to $38 million--with $3 million to be paid in cash and the remainder financed by the city over 15 years.

In addition the developers, Banneker Associates, who testified two years ago that they had "firm commitments" to lease about 1.3 million square feet of office space, said yesterday that all those commitments have collapsed.

The developers said they now want to build only one office building at a time, rather than seven structures within three years, as originally planned.

The revised plan outlined by Banneker yesterday to the Redevelopment Land Agency, the city's urban renewal agency, represents a major step in what has been a difficult, years-long effort by the RLA to redevelop the 10-acre site at the foot of the 14th Street bridge.

The prized parcel is the largest and virtually last city-owned land in the Southwest urban renewal project, which was started in the 1950s.

The Portal deal marks the third recent instance--along with Metro Center and Gallery Place--in which the District government has been asked to reduce the purchase terms for developers in order to spur development of long-delayed commercial projects.

The five-member RLA board expressed doubts and disappointment yesterday about Banneker's proposals, but postponed action on the deal until July 19, pending discussions with RLA's lawyers and with Deputy Mayor Ivanhoe Donaldson, who negotiated the proposed terms with Banneker.

"The board is not ready to jump on it and embrace the deal that is on the table right now," said RLA chairman Nira Long.

"We are asked to forgo a tremendous amount of our cash expectations to finance this," Long said, " . . . The question is, is this something that the city ought to participate in, and, if it is, how should the city participate financially?"

Long said the District has to resolve an important policy question of whether the government should "assume the risk" of essentially becoming Banneker's partner, because its quarterly payments from Banneker over the next 15 years would depend largely on the success of the project.

"The real struggle, the dilemma, is that the deal is so substantially different" from the original plans, Long said after the meeting.

Banneker, whose major partners are prominent developers Theodore N. Lerner and Melvin Lenkin, was awarded rights to the site in February 1982 after an intense competition with four other partnerships.

Major selling points of the Banneker proposal had been its claim that it could develop the land quickly because it had leasing commitments and solid financing to build an office-hotel complex valued at $355 million. It was later disclosed, however, that the leasing commitments were not quite so firm as first thought.

S. Lee Narrow, a spokesman for Banneker, said the revised purchase plan was prompted by the slump in the economy over the past two years and the downturn in the Washington market for office space. He said the $7 million proposed reduction in the price stemmed from site improvements the developers must make.

Because of the economy, several firms that signed letters of intent to lease offices canceled those plans, Narrow said. The major prospective tenant, Comsat Corp., has moved to L'Enfant Plaza to space vacated by its parent company. "If we had not lost Comsat, we would be building already," said Banneker architect Vlastimil Koubek.

A planned 434-room hotel on the site will not be built until Banneker constructs the office buildings on which the hotel would depend for much of its business, Narrow said.

RLA member Judith Jenkins said she was concerned because the city will lose the cash it hoped to use to pay off long-term urban renewal debts and to spur redevelopment of "problem corridors" such as H Street NE. Jenkins also said that the RLA may be threatening the "integrity of the selection process" if it agrees to major changes and conditions that were not available to Banneker's four competitors.