Offer your thoughts or opinion on the input financial query or topic using your financial background.

What is today's price of 15 000 Euro given 15 years ago?
What you are positioning as a loan was not a loan at all. Your father bought something to be delivered in the future. Your aunt does not want to deliver it, so she should buy it back at whatever the current market value is. What is the price that your dad believes her share of the inheritance is currently worth? Is that based on actual appraisals and some sort of objective audit? If so, your aunt doesn't have much of a case. If not, then she could seek an audit to bolster her bargaining position. How much did your aunt benefit from having a place to live for the last 15 years. Was that benefit greater than some larger amount of money at an unknown future date? That's probably why she sold her inheritance 15 years ago. Now that the inheritance looks like it is going to be available soon, she wants to trade back after having enjoyed the use of your father's money. That might be okay, but simply paying back the original sum with inflation, but without interest, doesn't seem fair to your father. She may not be able to afford to give any more than what she is offering, in which case, she might want to consider offering the original sum now and some portion of her inheritance as interest on that original sum. I'm not taking sides in this one. If it were one of my siblings, I'd be inclined to give the benefit of the doubt and take a smaller amount back if I felt that the lesson was learned (and if I felt that he/she would make wise use of my gift to him/her).  I have no idea what your father's current economic situation is, nor am I aware of any other baggage that might influence his feelings about his sister. It's as likely as not that money isn't really what is bothering him, in which case, the amount she repays may have little to do with bridging the divide between them. You might need to ask different questions in the Interpersonal Skills stack if you want to help your father feel better.