NO. I am fairly certain that he can buy the car, and sell it within X days and not pay any tax.The CA/inspector used the term "mileage credit" but I think he meant it more as an "allowance". Just as Sarafil and dkreidel described, the dealer will use them to waive repair charges but won't actually give any money back.
My brother is probably going to return his 335ic with only about 10000 miles on the 30000 lease. The car is an immaculate cream puff and I suggested that he sell it instead of returning it to see if he can get some money for all those unused miles. However, there's hardly much advantage left after you factor in the sales tax he has to pay to BMW to buyout the car, and then the next buyer would have to pay the sales tax again!
Do some research on this, but this is one of the bad things about leasing- it is a bet your use will be what the lease terms contemplate, and when they don't (over or under) you lose....