Debt deduction

Me: That thing cost 280
GB: Ya
Me: I paid 60 for auto
GB: Ya
Me: (giving him a 100), So I owe you 140 - 60 = 80
GB: Ya
Me: You owe me 20
GB: OK

I know that I don't know Maths, but can't believe GB didn't find anything wrong with it!

Of course, he does know now that I owe him 10 bucks and plans to use my line of thought in future transactions...