I had borrowed SR10000 for one year. But I could not pay it back because I did not have the money. After 20 years I have the money. Do I have to pay only 10000 or do I have to pay any more taking into consideration that the value of that amount after 20 years does not equal more than 1/10 of the value when I took the money?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

You should pay your friend the exact amount that you borrowed from him, namely SR 10,000. It is not permissible to pay him its value, as this is Riba (interest and/or usury). The scholars agreed in a consensus about this.

Allaah Knows best.