The first time I made a donation to the shelter I was helping, they said that I did not have to because I was already volunteering socializing the kitties and it was a great help, but they needed money so they accepted it.

Perhaps your vet has other reasons that she can't tell you. If she really believes it's too much, may be it's because she knows she would be able to set up her pratice without your gift and she prefers that you give it to someone else? I would accept her choice (it's just my opinion of course)