When Frank Mann, the former potato chip king and mayor of Alexandria, bought an acre of property on the Potomac waterfront for 10 times its value in the early 1970s, he was hoping to cash in on what many expected to be a bonanza.

That payoff seemed to be within his grasp until recently. Late last year city officials approved settlement of a longstanding lawsuit with the federal government over who owned Alexandria's 25 acres of riverfront. Agreements with Mann and about a dozen other private landowners were expected to follow.

Today Mann and the others are accusing the government of a turnaround, charging that the Justice Department is now demanding hundreds of thousands of dollars before relinquishing federal claims to their holdings.

Instead of the development concessions it once sought, the landowners say Uncle Sam is panning for gold on the shores of the Potomac. And there are few people in the city who believe Alexandria will gain in the end.

"It makes me ill," says Mann, whose plan for a restaurant, office and marina complex has already been approved by the city but can't go forward until he settles with Justice. "It's a complete reversal. What it means is that you wind up paying for the land twice, and what it really means is it forces your hand. If you pay double the money, you may have to double the development . . . or use cheaper building materials."

Justice denies it has changed tack, but acknowledges that money is now as much a part of its settlement talks as development concessions.

According to Justice lawyer James Draude, there is no formula for determining how much money will be sought for various properties. Rather, it's a matter of judgment. "It depends on our evidence and how strong our case is for each property," says Draude.

If settlement, either financial or in the form of development concessions, cannot be reached, the government will take its case to court. But that litigation is expensive and not the preferred course of action.

Alexandria Vice Mayor Robert Calhoun, who has both waged and watched the waterfront feud, says the financial settlements are something new, and that they could hurt city efforts to plan the waterfront. "What once was the tail has become the dog," he says.

"The thing is being treated increasingly as a real estate deal, rather than a means of controlling development on the waterfront," says Calhoun, the city's Republican candidate for mayor. "Up until about eight months ago there was a more principled basis for the dispute. We wanted to develop a plan that would preserve the historical and environmental aspects of the waterfront, and the argument over title was a vehicle to accomplish that."

Alexandria has zoning powers that should give it ultimate control over what is built along the shore, but the federal government's quest for money, says Calhoun, will actually weaken the city's hand. "Values are being built into the property that we have no control over. When someone pays a very large sum to settle, and we come along behind with zoning plans that don't reflect that . . . you've got a problem."

Take Mann's little acre, land that Mann once believed was "reasonably priced." He and two partners paid about $300,000 for it, and have paid taxes on it all along. To keep all of it, he estimates, he and his partners will have to pay the government $700,000.

As far as environmentalist and former City Council member Ellen Pickering is concerned, the public is back to "square one." Pickering, president of the Northern Virginia Conservation Council, has been in the thick of the waterfront battle since its inception, crusading with limited success to preserve the waterfront as a national park.

Pickering believes the settlements being negotiated amount to little more than sale of public land to private developers. It is a far cry from what she and others hoped for when they first asked the federal government to intervene almost 10 years ago. She is convinced that were the settlements to come before Congress, as they would have in 1972 in a bill introduced by then-Rep. Joel Broyhill (which granted blanket permission for private development of anything including gasoline stations), they would never pass.

"This is the Reagan administration," says Pickering, "they are business-oriented people. Mr. Interior Secretary James G. Watt and the others want to see this waterfront developed, and they realize that they can't give the land away, so they must extract something. The lawsuit has been a success in that it has made everyone face up to the fact that the federal government, and the public, have a claim to this land. The failure is that now the land is being lost, and the public is not receiving anything in return."

Though the waterfront battle was joined only a decade ago, it's been simmering since the early 19th century, when what was Alexandria County and the federal government traded about 25 acres of riverfront land. In the process they clouded the title to some of the waterfront.

The land was bought and sold over the years as private property, and its actual ownership was not contested until 1973, when local environmentalists seeking to block development reminded the federal government of its claim.

Until recently, Alexandria's was a working waterfront, a dumping ground for industrial waste and home for pile drivers, barges and weatherworn rivermen. Nearly all of the families who own the property bought it years ago, and used it for things such as fertilizer manufacturing. The Washington Post, for example, is a major landowner on the waterfront and receives newsprint on ships from Canada at two terminals.

Largely ignored by the city, and most of its citizens, the waterfront has always been an ugly duckling. As Washington and its suburbs have grown, and river life has faded, the ugly duckling has acquired a certain allure. Even those landowners who are nowhere near settlement say they talk to one or two interested developers a week. "They look closely, become aware of the title problem, and then they go away," says Randall Norton, whose family operated a rendering plant there for years.

There is some development activity on the waterfront now. Much of it foreign financed, mostly office space, none of it on property involved in the lawsuit. And the commercial space is filling slowly if at all.

The old United Way office building, on the north end of the disputed waterfront, was recently purchased by a Kuwaiti couple. Trans Potomac Plaza, a five-building office complex slowly rising on three acres nearby, is being built with Dutch and Swiss money. Though the first building will be ready in December, no leases have been signed. The same developer has an option to buy the adjacent 16 acres from the current owner, but he must settle with Uncle Sam first.

Another developer recently abandoned an office project nearby, though its site plan, designed by a prestigious Washington architect, had already won city approval. The reason: insufficient funds.

Clova Demaine's family has run its funeral home in Alexandria for 140 years. She has no property on the waterfront, but she, like many in the city, has dreams for it. Her idea calls for a civic center and theater for the performing arts.

But she sees those plans evaporating as the government ups the ante. "We'd even figured out where it would go, who might donate the land. It would have been like the Kennedy Center, only much smaller, of course. We could have become the capital of Northern Virginia," Demaine says wistfully.

Demaine has no idea if she will ever see such a building, if there will be room on the waterfront for such things, if large settlements will make the land too valuable to donate. These days question marks loom as high as cranes on the waterfront.

"I'm very concerned," says Demaine. "I feel Alexandria's going to sell itself right down the waterfront."