Flint has received $50,000 from some of the thousands of taxpayers who filed a state return but not a city return.
Emergency manager Michael Brown says 6,000 people have been identified, and letters so far have been sent to all but 800. He said "shame on us" if Flint doesn't enforce the law.
Brown says the city is willing to agree to a payment plan if paying a lump sum is a burden. He says pursuing the taxes could add up to a windfall of more than $1 million for troubled Flint.
Flint has been operating under an emergency manager since 2011.