We have a software we use for our subs and they are required to submit photos properly, and timely in order to get paid. And any pay request has to be turned in on Thursday and we will review their work and any work that is 100% complete we will pay 80% of it out on Wednesday. Any partially completed work can pay out a percentage of the full but it is based upon the level of completion of the correctly submitted work and it is 100% at Elysium Property Services discretion what that percentage amount it is.. The remaining 20% will be paid out at the end of the project and the work passes the buyers inspection. Which is normally within 1 week. If it exceeds a 3 week time period the other 20% will be paid regardless. The sub-contractor is responsible for any "punch list" work that comes from the final inspection by the buyer.
You should contact an attorney to help you put together a subcontractor agreement that protects you and the subcontractor. I've done quite a few of those, and you could also find a local construction attorney who could assist you.