If the item was specifically donated for the auction, the employee has probably caused the charity to violate its understanding, i.e. contract, with the donor, but it is also unlikely that the donor would sue to enforce the agreement.

It is more obviously, and perhaps more aggravating to the donor, an ethical violation.  The donor has given a valuable item and presumably hopes that it will go for a better price at the auction.  Whatever the employee may have paid for it, the early purchase cuts off the opportunity for a greater payment at the auction, depriving the donor of the satisfaction of giving a more valuable item and the charity of increasing its income.  Without the prior approval of the donor and the proper decision-maker at the charity, it is the type of conduct that does not reflect well on anyone.