Outbid (one who has an agreed house sale) (6)

(Other definitions for gazump that I've seen before include "Successfully outbid someone with already accepted offer on a house" , "Raise price of property after agreeing a lower sum" , "After agreeing a lower price with one person, accept a higher one from another" , "Offer more for a house than (an earlier bidder)" , "Raise the house price after accepting an offer" .)