I'm going to ask my ex to pay his debt
Is this why you are angry at him.
Yes, I really want him to pay his debt
If your ex is in debt with you then it is fair for you to want him to pay his debt. You holding your ex accountable is not your problem, but more of his instead.
He said he is unable to pay his debt. Thanks for your opinion, bye.
Maybe you can ask him to set up an affordable payment plan. That way you can get back small amounts at a time instead of nothing.