My JV partner wants to grandfather our old split on a new deal because we used the same buyer list

Your instinct to reset the agreement on each deal is correct as a matter of structure. A closed deal creates no ongoing entitlement to future deal terms. The prior 50/50 was consideration for that specific transaction, and a buyer list that existed in June does not automatically earn a fee share on a Sapulpa contract that has not been signed yet. Those are two separate value propositions.

The assumption doing the most work in his argument is that the buyer relationship is the scarce resource. It was, last time, because he found the buyer and you presumably needed him for disposition. Ask yourself honestly whether that is still true here. If his twelve-person list is stale and you could cold his buyers yourself, or route through a different disposition partner, his leverage in this negotiation is mainly psychological, not structural.

The risk you have not mentioned is time. You said the seller wants out by end of month. A two-day silence from a partner, followed by resistance to a new agreement, is a pattern that can eat a deal. If he stalls through the weekend and you lose the contract window, no split percentage matters.

On the 60/40 proposal: the split itself is defensible given you sourced the lead and still carry disposition risk if he contributes nothing new. But make the agreement deal-specific, address by address, and spell out exactly what each party is delivering on this deal, not what they delivered on the last one. Contribution clauses matter more than the ratio. A licensed attorney in Oklahoma should review any JV agreement before you execute, particularly around assignment and earnest money provisions, since wholesaling regulations vary by state and I cannot confirm current Oklahoma requirements.

One practical question: do you have a signed purchase agreement with the Sapulpa seller already, or are you still in verbal territory with them?