The Financial Permeability
The Big Bang Theory Season 2 Episode 14 (02/02/09)
The episode opens with the guys discussing plans for dinner and a movie, but when they realize they'll never be able to find a movie theater and restaurant that are acceptable to Sheldon with a time that works, they leave him behind. Sheldon completely understands and when he goes out to buy himself some Red Vines he reutnrs home to a fleeing Penny that he hides in his apartment.
When Penny tells Sheldon she's short on rent money, Sheldon is more than willing to give her some money he keeps hidden in his can of snakes. Despite how awkward lending money to friends is, Sheldon proves this is one quirk he doesn't have! Sheldon lends her the money and establishes no due date and never harasses Penny about it. However, Penny soon becomes very weird and Leonard picks up on the fact that Sheldon lent her money.
Leonard goes over to Penny's to help her figure out how to cut costs and pay back to Sheldon. Leonard finds out that her ex-boyfriend Kurt still owes her $1500 from when she paid his court costs for peeing on a cop car. Nice. Leonard gathers the nerd troops as his backup and heads over to Kurt's place to try and collect. He goes over, his backup bails, and he manages to leave unharmed and without $1500 but with the words "Kurt Owes Penny $1500" on his forehead.
The episode ends with Penny paying Sheldon back. Apparently Kurt decided out of nowhere to pay Penny back. Leonard does not take credit and now Penny thinks Kurt is a nice guy and is having dinner with him. Oh Leonard, you unsung hero. Well, except for when Sheldon sings for you.