It's not mentioned in the contract at all. Can the seller legally get out of selling the house to us if we don't take over the lease?

it's a HAFA short sale through BofA and was submitted as an ARASS.

Would the accepted short sale prevent her from walking away? She stands to make $3k through the HAFA program if she does the short sale.

(We have a dual agent and he's not helpful but motivated by the double commission)

Thanks!