That sucks! Karma will totally come back to bite them in the ass!

Hate to burst your bubble, but karma is not real.

From OP's comment, it seems like it was a misunderstanding. There's a lack of information, but it seems like neither checked with the other before acting. OP canceled their plans, while their partner didn't. Maybe it's one of those 'shit sorry' moments.

Tell him it was a bad schedule and timing

Time to renew your previous plans!

Some people

once told me the world was gonna roll me. I ain't the sharpest tool in the shed.

Go away, Shrek.

*body

Time for a nice shower before hitting a charting system of ideas and coffee breaks

Cancel the plans and go out then!

That situation sucks, but his personal suck level depends on whether he knew you were expecting him to sacrifice his evening before you started cancelling things.

I'm curious- did he KNOW you both were going to work on a project for four extra hours or did you just go ahead and plan out the evening and hoped to spring it on him?

Time to confront the problem!

Time to do most of it yourself and prove that you did so it makes you look better.