Charge him another £100, that’ll learn im. Then sleep with his wife.
That’s not really a moral dilemma but more a contractual discrepancy. How have you earned this £100? Is it not just a dibs fund?
You know you should give it back, you just want your ego opinion aired so strangers can call this chap a muppet.
Here you go: “he’s a muppet”
If you have spent any money base on the assumption of sale (courier arrangement to him, specially requested parts etc) then ask for that back, but I can’t see the justification in pinching his money (or wife).