Well to totally nerd out here: The scene you are talking about is only in the new version of the film. Anyway, Jabba says he wants his money and Han says he will get it. Jabba gives him more time and then Han steps on his tail. Hysterical (not). Its in Episode V in which Han is telling Leah he has to pay his debt and she says he should stay with them. The rebels gave him his reward. I remember thinking, what's the big deal, let him go pay and then he can come back. Or send someone else to give it to Jabba or paypal it for crying out loud!I may not be remembering this in the right, but while Obiwan did give Han Solo some money up front, I thought or it could have been not enough, or he may have needed it for operating expenses, with the intent of paying Jabba after the big day when they arrived in Alderan. In the updated Episode 4 where Han meets Jabba in the hanger before they leave for Alderan, they discuss his debt, but honestly I don't remember what is said.
Here's where he says he is going to pay off his debt, kind of..... He doesn't actually say he has the money right there and then.....