In Detroit, where good deeds rarely go unpunished, Mayor Dave Bing is still trying to cobble together a deal to keep General Motors Corp. in its downtown Renaissance Center headquarters.

Problem is, how to do it without gutting the very agencies that run the city's business attraction and retention efforts, the Detroit Economic Growth Corp. and Downtown Development Authority?

Here's the GM-RenCen dilemma.

A week ago Friday, amid growing hysteria over whether GM might vacate the RenCen and leave Detroit as part of its crash slim-down plan to enter and exit Chapter 11 bankruptcy rapidly, Wayne County Executive Bob Ficano made Bing a tantalizing proposal.

Wayne County could offer its one remaining renaissance zone designation to the RenCen, thus exempting it from a host of property, income and utility taxes, saving GM $10 million or more.

Bing and GM President Fritz Henderson liked the idea. Even President Barack Obama called Bing to voice support for keeping GM downtown.<

Hallelujah, GM would stay put!

But alas, this is Detroit, where nothing comes easy.

Soon some unintended consequences of the ren-zone plan reared their ugly heads. Taxes that would be waived to help GM are the very taxes that bankroll the DEGC and the DDA, which in turn help finance city projects large and small, from the Comerica Park and Ford Field stadiums to Campus Martius office buildings and residential lofts.

Art Papapanos, vice president of the DEGC and officer of the DDA, said Friday that staffers were still calculating the exact impact, but he estimated that one-third or more of his agencies' annual budgets come from RenCen taxes. "That's a big chunk," he said.