What happens if I can't pay my mortgage?

Does the bank take the building along with my down payment?

Or does the bank put the building up for sale and whatever it sells for the amount pays what I owe to them and the rest I keep? For instance I owe them $400,000 and it sells for $700,000. They get $400,000 while I get to keep the $300,000? Or do they get all $700,000?

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