What is the ruling on the sum of money that entered my account and has no known source, and must I pay it to someone, especially since more than 12 years have passed since this occurred?

The amount that entered your account ($500) is part of the transfers sent to your colleague ($1500 in 3 transfers), unless proven otherwise. You can trace the source of the transfer to verify. If you confirm it is not for your colleague, then you may dispose of it, and you must hand it over to its owner whenever you identify him.