Only a genius can solve this brain teaser!

Someone forwarded this brain teaser to me over the weekend and i must confess, it really messed with my mind.


All those who I have sent it to have come up with different answers and they swear theirs is the correct one.

Lets see if you can get it right:

Someone steals your N500. Same person owes you N100, so he uses the stolen N500 to pay you his N100 debt and you give him N400.

How much did you lose?