The places I’m familiar with have paid commission upon receipt of payment. So if a client pays the contract up front, then the trainer gets paid up front. EFT – commission gets prorated when client pays.
I offered this to my clients a while back. The majority didn’t want to it. They preferred to pay me directly. So, I didn’t do it. I am sure that eventually, I will have enough clients that want to use this, that I will go ahead with it.