God, I hate people like this - entitled, can’t follow simple process, blame others for their incompetency, go all Karen when the ball is actually in their court.
I’d just keep doing a copy and paste of what he needs to do to get his money. I recently discovered the concept of “Don’t JADE” - justify, argue, defend, explain. The idea is that once you’ve given someone a reasonable answer or boundary, you don’t keep trying to convince them that you’re right. Every extra explanation gives them another detail to challenge and turns the interaction into a negotiation.
This is how I’m trying to manage difficult people without getting myself worked up, stressed, apologising when I don’t need to or explaining the same thing over and over again. Because, at the end of the day, “life is too short”.