Share your insights or perspective on the financial matter presented in the input.

What is today's price of 15 000 Euro given 15 years ago?
With no written agreement in place, the "right" rate is whatever both parties can agree to. I could argue that I could have invested the money in S&P 500 index funds and made about 9% annually over the last 15 years and the 15,000 would have been over 40,000.  The "fair" rate would be whatever rate of return could have been expected from whatever your father would have done with the money otherwise - keep it in a bank account, pay off debt, invest in the market, start a business, whatever.  Your father has the benefit of hindsight to know what would have been a good use of the funds over 15 years. Using the rate of inflation results in effectively a zero-percent loan in real interest terms (meaning no profit was made, just accounting for the time value of money). Both parties need to either decide on an amount or equivalent rate, or decide if squeezing the other for a few thousand Euros is worth the strife.