Obviously if the donor expects that the full fund will immediately be spent, that's another issue - and it must be spent. In the context of a "donation fund" instead of a simple donation, where the donor knows up front, I guess he wouldn't mind if that means that the donation can actually be doubled or tripled. To ensure that the spending will occur, you may put a time-limit in place ("if the price doesn't reach the desired targets, the amount will be spent in 3 years time at market prices"). If that's too complicated for donors, or they might think it sounds sketchy, you can avoid it altogether :D