money stumper

I saw a pair of shoes I really wanted
So I went to my parents and borrowed $250 from my mother and $250 from my father
Went to the store and saw they had a sale instead of 500 it was now 450 so I bought it that left me with 50 in my pocket on the way home I met a friend that borrowed from me 30 so now I'm left with 20 I go home give mother 10 and father 10 so now I owe each 240 right

Now let's do the math 240 +240 = 480 I have another 30 by my friend that's a total of $510 I only borrowed 500 where did that extra 10 come from
Let's see who can figure this out