Lets be realistic about it, people jumping on the band wagon suggesting its about integrity etc, next someone'll suggest getting a posse together so we can have a lynching. Its just an error, a mistake a misjudgement.
The guy said that he had just recieved your invoice, so why dont you just invoice him when the job is done on the spot, there and then and he can take that as his 'slip'.
You get to invoice him immediately he gets his slip and no-one is any the worse off.
I can be diplomatic when I try