Sold some gp but didnt get $ as agreed

So i was supposed to recieve a payment of 528$.
He claims it was 528$ by the time he send.
Yet my wallet shows it as 520$.
This could be because btc price goes up and down.

But when we agreed to 528$ - Shouldnt i be recieving 528$.
If it crashes afterwards it should be on me.
But i never had 528$ from the transaction.

Who is right or wrong?

 

Sorry im new to using the forums..

But it was just a general question.

Anyways he paid up, so i'll edit his name out.

Would still be curious to know the answer tho.