You should have a vendor contract in place before they start work - if they're alraedy started working, then you need to put it in place immediately.

The agreement should lay out pretty much everything - each parties (i.e., you and your vendor) responsibilities, milestones, payment schedules, rework guidelines, etc. You said you're going for a patent - are they going to own any of the patent rights? This should be specified in the agreement. I've seen many a times where this stuff was not specified and then both people claimed rights.....

Good luck!
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