How do I return an inappropriate gift without offending the giver?

I face this issue, with my staff, in a counseling center at which some clients give gifts that sometimes go too far. I think you should return the money with the explanation of your professional context. Let her know your appreciation for her participation and for her thoughtfulness. Let her know that external controls require the return of the cash. Ideally, the use of the external structure should neutralize personal feelings. I suspect that our ethics and regulations are quite similar to yours, and they need to be respected. But the public often doesn't know about them, hence the context and explanation.