Retroactive renegotiation after close is a well-known pressure tactic in service work, and it shows up in wholesaling-as-a-service more than people expect. The buyer received what he contracted for, you delivered inside the agreed parameters, and the contract gave him no adjustment right. That is the whole situation in one sentence.
A few things that matter here:
Your written agreement is your position. If the fee, the deliverables, and the payment terms are documented and he signed, the conversation he is trying to start has no legal footing. Whether it has practical footing depends on whether he has already paid or whether there is still money he controls. That distinction changes everything about how you respond.
If payment is already cleared, you can acknowledge his concern, decline the adjustment, and let him decide whether he wants to keep working together. If he is holding funds, get an attorney involved before you respond further. I am not giving legal advice here, just naming the fact that an unpaid fee plus a renegotiation request is a different situation than a paid one. A real estate attorney in Oklahoma is the right call if any money is still outstanding.
The part people in your position sometimes overlook: his comment that "the numbers looked good at close" is actually a compliment to your sourcing. He is not saying you failed. He is saying he made money and wishes he had paid less for the work that helped him make it. That framing can help you stay calm in the conversation.
Document everything from here in writing, even conversations that start verbally.
Was the full fee paid at close, or is some portion still pending? That changes the practical advice considerably.
