You don’t say whether the vet is an officer, director or employee of your rescue, but he is probably a “disqualified person” for excess benefits tax purposes because he is in a position to exercise substantial influence over your activities.  Therefore, making a gift of $500,000 to him (or his wife) would most likely be considered an excess benefit transaction unless it is treated as compensation (and he pays income tax on the benefit).  (See Ready Reference Page:  “Charities Must Avoid Excess Benefit Transactions.”)

You can probably avoid that situation by treating the investment as a leasehold improvement to a property that you actually lease from the vet (and/or his wife) for a fair rental value for a term at least as long as the useful life of the improvements.  The lease should also provide that if it is terminated before the end of the useful life of the improvements, the landlord should repay you the undepreciated value.  There are issues of valuation in this set-up, but at least you should be able to avoid an excess benefit tax situation.