Purchased vacation

A question for those who allow their ee's to purchase additional vacation time to supplement their earned vacation. Our policy states that at the end of the year we will reimburse up to 40 hours of vacation, purchased vacation, and personal time. All hours exceeding 40 are forfeited. This past year we changed our policy to allow ee's to purchase up to 64 hours of additional vacation, but did not change the reimbursement policy. So, if an ee purchases 64 of vacation with the money obviously coming out of their own pocket and they do not use it, we reimburse them only for 40 hours - they lose the other hours. What is your collective opinion, could we be in trouble for not reimbursing all of their money for unused purchased vacation?
Comments
We only allow 40 hours to "rollover" each calendar year with all other time vanishing into thin air. But our state does not have laws requiring us to pay out unused time upon termination, other than to follow your own policy. What you do for one you do for all.
Another way to look at it, you lease a car for 30 days and let it sit in your driveway. Never used it, but at the end of 30 days, you still give the car back. It was always available to you, but it was your choice to not use it.
I personally would try to find a way to either reimburse or allow the unused purchased hours to rollover. A bit more difficult to track, but to use the F word, it seems more fair.