I wasn't able to find anything specifying a minimum payroll. I did find this:
Marlins' new look, stadium not fooling anybody - MLB - Yahoo! Sports
which makes it pretty clear that South Floridians are getting fucked in the ass by this deal.
It's an awful deal for the city.
They're now in dept booku bucks. To top it all off the SEC is investigating the whole situation, and just subpenaed a crap ton of financial information from both the city and the Marlins org.