Am I sinful if I spend a sum of money that was added to my account, whose source I do not know, while I suspect that a friend of mine transferred it through my account to deliver it to him, and he denied that?

This money belongs to an unknown owner, and you are not permitted to take possession of it. Rather, you must contact the entity responsible for your account to ascertain its source. If you are unable to identify its owner, then give it away as charity on their behalf.