Gazump
Etymology
From Yiddish "gezumph," meaning to swindle or overcharge, which entered British slang in the 1920s meaning "to cheat." In the 1970s British property boom it narrowed to its now-standard sense: accepting a buyer’s offer and then, before contracts are signed, taking a higher one. It fills a gap English otherwise lacks — a single verb for this exact betrayal.
verb · /guh-ZUMP/
To raise or renege on an agreed price at the last moment, especially in a house sale after accepting an offer.
What it names
A precise name for a specific bad-faith move — snatching a deal away by demanding more once the other side thinks it is settled.
Example
They were days from exchanging contracts when a cash buyer gazumped them by twenty thousand.
Related idea
To welsh is to fail to pay a debt you owe; to gazump is to break faith on a price by demanding more than agreed.