Michelle,
To me, what you are saying is exactly like renting your list out because the sponsor is paying you cash and receiving consideration.
In most jurisdiction now, you can only send email on behalf of others to your own list, meaning the sponsor could never have direct access to the list....unless of course, the person knew about, and explicitly, granted permission for you to hand over their email.
**I'm not a lawyer, but I would agree with you that this sounds like it is against your firm's stated policy as well as several laws.
So no, you should not do it.