The Benefactor Factor
The Big Bang Theory Season 4 Episode 15 (02/10/11)
The university director invites the guys to a fundraiser party, but Sheldon refuses to go. Amy convinces Sheldon that if he doesn't go, Howard and Raj will have to convince the benefactors that their research deserves funding. Sheldon decides to go, but offends the donors, and the director asks him never to come to another fundraiser again.
The largest benefactor, Mrs. Latham, takes an interest in more than just Leonard's work. She insinuates that she'll give him funding in exchange for a romp in the sack, but Leonard resist, while Sheldon works every angle to encourage him to go through with it.
She eventually tells Leonard that she'll give him the money regardless of what happens, but says that if he will give her a chance he'll remember it for the rest of his life. Leonard decides to heck with it and gives in.