I've also been in the equally unenviable position of having to pay for someone who completed work according to a scope that was too vaguely defined. The guy I contracted to do the work did the work according to the broad outline I originally set out.

Being young & stupid at the time, I blamed him for thinking he took shortcuts and refused to pay him all of what he was owed at first. But it was my fault for not being more specific in the original job. I eventually apologized and paid him the rest of the money. Lesson learned.

I believe design work is especially prone to this. But none of this is an excuse. If you have a signed contract, you honor it. Period.