$850? You are getting off easy. Give him the full amount (or what he hasn't paid you minus the full amount) and be done with it.
If you took a chunk out of my drive way, I'd want the whole thing replaced. It will never look right, and more likely to get potholes with water between the cracks - even if you cut the section out and repave it. If he is happy with a half-way repair from the company of his choosing, pay it. Who cares if he does it cheaper and keeps a couple of hundred - if $850 is the cost of the job, that is what you should pay in my opinion.
If you took a chunk out of my drive way, I'd want the whole thing replaced. It will never look right, and more likely to get potholes with water between the cracks - even if you cut the section out and repave it. If he is happy with a half-way repair from the company of his choosing, pay it. Who cares if he does it cheaper and keeps a couple of hundred - if $850 is the cost of the job, that is what you should pay in my opinion.