Converting money value in gold before lending

This question came to my mind in a discussion. Suppose a person asked me to lend them money over a long period (**10000$**. Repay after four or five years as an example). I want to know whether this idea is permissible or not : > I convert the money amount (in that very moment) in Gold ( **120 grams** for example), and lend that person the money they want on the account that in five years they will give me back what 120 grams of gold cost in that time in future.