I'm assuming he means the use tax, i.e. sales tax paid after the fact by a purchaser. A car registered in Washington less than 90 days after purchase will require a use tax to be paid when the car is registered, however any sales tax paid at the time of purchase or initial registration in another state may be credited against that tax I believe. So make sure you have the receipt from the Michigan DMV that shows how much sales tax was originally paid and you may not end up owing as much. I'm not 100% certain about this but that is how I understand the process works.