IF DEVELOPMENT of downtown's prime Metro Center site follows the provisions of the city's unusual proposed sales agreement, the construction sign could well say, "Your Tax Dollars at Work-- God and the Market Willing." Having cut millions from its asking price for the land last year, the city government is now proposing to sell the site to developers Oliver T. Carr and Theodore R. Hagans in parcels--in exchange for a share of uncertain profits in the distant future.

Instead of $37 million, as offered last year by the developers, or $51.6 million, which the city was demanding then, the price would be roughly $14.5 million in cash at sale time and $14.5 million more in 30-year notes--with the city sharing in future profits from the complex. The question for the city and its taxpayers--and for a vote scheduled today by the city's Redevelopment Land Agency--is whether these terms are right.

Maybe not, says Robert Siler, a consultant hired by RLA to evaluate the proposal. The agreement says the city would eventually receive $29.7 million as its share of profits from the office and hotel complex planned for Metro Center, atop Metro's busy subway station at 12th and G streets NW. But Mr. Siler estimates that the government would not start receiving its money until 2008 and that the take, adjusted for inflation, would be the equivalent of only $2.9 million in today's values.

Disagreeing, Deputy Mayor Ivanhoe Donaldson has estimated that the city could begin receiving its share as early as 1991. Possibly so, says Mr. Siler, if --and here's the gamble--revenues turn out to be higher and costs lower than estimated by the developers. Mr. Donaldson has argued that the city's participation in the cash flow of profits "involves a risk based on future market conditions. We believe the risk is a reasonable one."

RLA may believe it, too. But before knocking on wood and rubber-stamping the agreement, the agency should consider some modification to deliver more money up front to the city, in line with Mr. Donaldson's estimate of a 1991 start of profits for the city. This is a prime site in the heart of downtown Washington, and the city could well hold out for a better and faster share.