Help! Background - I’ve been paying a mortgage for a family member for the last 30 months to keep them from getting foreclosed. The verbal contract (Texas and legally recorded) was that this would be considered a 0% interest loan that would be repaid from sale proceeds. Now that the sale is about to close, family member thinks the mortgage money portion is “too much”, even though I have cancelled checks for every mortgage payment made. Some family members share in this opinion. The perception is that I’m “charging too much”. We’re obviously a bunch of math geniuses in my family.

What I need help with is how to calculate loss of purchasing power from month of payment until now for every single monthly mortgage payment made.

I’d like to do the same to show the loss of investment opportunity with each mortgage payment vs the growth of my 401 (k) over that span of time.

I have monthly inflation rate and the growth rate of my 401(k). What I need is a formula to put it all together and show how my act of kindness has personally cost me over this span of time.

Any help would be greatly appreciated.