Qitzur Shulchan Arukh – 62:15
One who has given even only a little money on a purchase, or has just marked the object in the presence of the seller, or the seller said to him: “Mark your purchase”, even though he has not formally bought this object, in any event any one who goes back on it, whether the buyer or the seller, has not done an act becoming to a Jew, and is obliged to receive a “May He who took payment”. That is, he is cursed in court and they say: “May He who took payment from [ie: punished] the people of the flood, and from the people of disunity [the generation of the Tower of Bavel], and from the people of Sedom and Amorrah, and from the Egyptians whom He drowned in the sea, He will take payment from one who does not keep his word.
A continuation of the theme we began two days ago.
62:13 established that it is evil to break the promise of a deal to chase a better one. 62:14 continues that someone who serves as a proxy to accomplish the deal, so that there is even no deal promised yet, is devious in trying to thwart the one who sent him. Here we see that someone who someone who took some action to initiate the deal — but again, the sale isn’t yet complete — who breaks that deal is formally cursed.
Notice the text of the curse explicitly ties the concept of Divine Punishment to that of a metaphysical “repayment”.
One’s word must be binding; and as we noted in 62:13, it does not depend on whom that word was given to.