Is it obligatory for someone who left a nightclub without paying the bill during a fight, and has since repented, to pay the money now?

The repentant person must return the money to its owners if it was acquired through permissible means according to Islamic law. However, if the money you owe resulted from a forbidden act, such as entering a nightclub, the owner of the club is not entitled to this money, and you must give it away as charity. If the court obliges you to pay the amount, then pay them, and you are not required to give it as charity.