I'm pissed that my friend is charging me interest on money that he loaned to me
Did he tell you he didn't expect interest when you borrowed the money?
Yes. He lies so often
If you agreed to the interests beforehand then you should pay him back the interest. If he didn't specify that before then it's not fair.
I don't have much money, so I can't really afford the interest.
Try reminding him that he told you the loan would be interest free. Explain to him that you really don't have the money for the extra cost. Hopefully he'll understand since he's your friend.