In the third party financing addendum for credit approval, what should be the cap on interest rates?

In the third party financing addendum for credit approval, about 90% of fixed rate loan with PMI that serves as a Market rate that should be the cap on interest rates. It is like a contract wherein the rate must be where the lender and the buyer has agreed upon and should not exceed this established rate between both parties. It is not useful for when buyers will pay money and the seller will finance for a a particular goods.