It is rude to not pay your obligations.
Jack was down on his luck and John loaned him 100 bucks to pay his rent, and now Jack has recovered financially and John is asking for his money back.
Jack needs to decide how to respond to John.
Jack pays John back the 100 dollars he owed him.
Jack tells John he's going to have to wait because Jack's enjoying having money again.
John appreciates Jack's timely repayment and tells him if he ever needs help again in the future he can feel free to ask him.
John stops being friends with Jack because Jack is unreliable.