We did not make a contract but took an oath. He took screenshots but I didn't. I think his username had "Owl" in it.
All I can recommend is when you are online type /seen owl that might generate a result if the user is online. Otherwise you will have to wait for him to make contact with you with regards to the loan.
Hey thanks sorry I was going to pay you on July 5th, but I forgot your username! XD. I'll pay you back ASAP! I have the money don't worry!