Bob Duffy is taking a $50-million stand on principle. The Rochester mayor says he won't sign an agreement with the county that's designed to preserve the city's share of sales-tax revenue.

"This is my decision on behalf of the city," Duffy says. "I just chose not to sign."

For many years, Monroe County has shared sales-tax revenue with the city, towns, villages, and schools through the Morin-Ryan Plan: a tax sharing that is written into state law

When the county bought into the state's intercept plan - letting the state take some of Monroe's sales-tax money in exchange for picking up the county's Medicaid tab - county officials said they would ensure that most of Monroe's sharing partners did not lose sales-tax money. They would do this by signing funding agreements with each municipality. The county did not offer the same deal to suburban schools, which sued and lost. The districts are appealing the decision.

Duffy says that he appreciates the county's position - Rochester, too, has had to dig itself out of a deep financial hole - but that he is "philosophically opposed" to the intercept legislation. He says he has told County Executive Maggie Brooks as much.

"Our belief is that although the city is held harmless this time, it's an annual thing, and there could be other administrations that make different decisions," he says. "I respectfully declined to sign."

There is ambiguity, Duffy says, over the legislation's intent, and the State Legislature should clarify it. The county says it only has to share what's left of the sales tax after the state takes it portion. Others say the county has to share the full amount collected before the state takes its cut.

In addition, state lawmakers should grant Rochester an exception to the intercept legislation because of the pre-existing Morin-Ryan agreement, Duffy says.

"You cannot unilaterally change sharing agreements," he says.

Duffy says he realizes it's a high-stakes stand - one that could cost the city about $50 million if the county chooses to cut the city out of the sales-tax pie altogether.

"My belief is you stand by your principles," he says. "You disagree with something, you do it. You take a chance on incurring other pain elsewhere."

Rochester could fight for its $50 million in court, Duffy says, but that's not the way he prefers to go. He says he prefers discussion and negotiation.

County attorney Dan DeLaus said at a meeting earlier this month that if a municipality declines to sign an agreement, the county will send the money anyway.

"If they didn't want it, they could return it," he said.