I owe someone money but i am not finding him to return him his money. What should i do?

First you have to make a few public announcement in the press or on the radio. When you are convinced that the person would not come or say 3-4 months time elapsed after announcement, then give the amount of money that you owe in sadaqah in favour of that person. The sawaab will go to him. Sadaqah is meant to someone who is poor. But if ever that person comes later, then still you will have to pay him.