Need Help Calculating Interest Rate

I have a series of payments where the monthly payment is the larger of one of two numbers. One rental amount is calculated using the Calc PMT function based on the value of the asset as PV, but the other rental is a percentage of sales and I have no way to assign a PV or a Rate.

How can I calculate the interest amount (similar to the Calc IPMT function) and the principal amount (similar to the Calc PPMT function) when all I know is the monthly (or annual) payment, the NPer, the Period, the PV and the FV, but not the Rate? Is there a function for this, or a formula I could use?

Any help would be highly appreciated.

Thanks