paying an old bill plus interest

A young boy took a girl out for a Valentine's dinner but found that he was $40 short when it came time to pay the bill.  He spoke with the chef, Claus

Hjortkjaer who pulled $40 from his wallet and helped pay for the meal.  The boy promised to return the money.  That was in the 1990's but just last week the now grown young boy walked into Chef Claus' new restaurant and paid the debt with interest.  $100