In Monopoly, what happens if the auction winner cannot pay his/her bid?
The player who cannot pay has just properly incurred, according to the Rules, a debt to the Bank that cannot be paid. That is the definition of Bankrupt.

If the player is unable to relieve the bankruptcy, then all their assets go to the Bank, which then auctions off all properties to the remaining players, as per the Official Rules for Bankruptcy.

The property originally landed on is part of the Bankruptcy Sale of the Bankrupt player's properties.

Capitalism is harsh boys and girls - no second chances in Monopoly.