Parable of Two Sons

Father and son by ~Gloredel
Photo Credit: Father and son by ~Gloredel

There was once a father who had two sons who he loved very much. He fed them, clothed them and paid for all their needs until they were both grown up.

One day, the father as he was slowly becoming advanced in years, decided to go into business. He used his savings and borrowed from lenders. When more funding was required, he borrowed some money from his two sons as well.

Unfortunately the business failed. But the debts still needed to be paid. Eventually the father paid of all his debts.

However, the eldest son believes that the father should pay interest for the amount owed.

Should the father pay the eldest son as requested
Should it depend on any terms that the father and eldest son had discussed prior
Should the youngest son pay the eldest son the amount owed
Or are there any other alternatives?