Blood Feud

Season 2, Episode 35

Homer discovers that Bart has a rare blood type that can save Mr. Burns' life, Homer convinces Bart (who was the only Springfieldian that has the blood type) to give blood (in the hopes that Burns will give the Simpsons a cash reward), but when all Burns gives Bart is a "Thank You" card, Homer writes a nasty letter to Burns which ends up getting sent to him by Bart.