If you cave this time. Next time he will say " last time was only 50 how come it's a hundred this time"
So the way I see it, you can take the $50 . "cause fifty bucks is better than nothing. and don't deal with him again
Or you can meet in the middle and charge $75 and don't deal with him again.
Or you can stick to your guns and ask for the $100, and don't deal with him again.
You can say "oop's,I accidentally deleted them" ( my favorite ) and he won't want to deal with you again.
Promises of future business are worth absolutely nothing.
Or you could charge him 100 this time with the promise of 50% off on the next job. which of course you will mark up 300% then he will end up paying 150% on it
The possibilities are endless.
Have fun with it, life is too short to deal with idiots (unless you can take advantage of them)