I was bidding on a free class during a charity auction and it got down to a bidding war between two of us, The person who donated the class asked if we just wanted to split the win. We both said yes and we each paid half of the highest bid.

The other guy went to register for the class but he is in a dispute with the person having the class and I am just confused. The full price of this multiple day class is $1800. The highest bid was $800 so we both paid $400 to auction.

When registering now for the class I expected to pay half which is $900, however the other guy is saying it is only $500 we owe because we already paid $400 through the auction. Which one of us is correct?