Qitzur Shulchan Arukh – 62:13
One who is pursuing an object to buy or rent it, whether it is real estate or goods, whether from a non-Jew or from a Jew, and has already agreed on the price, but before they can close the sale, another comes and bought it or rented it, he is called a wicked person. However, if they have not yet agreed on the price, but the seller wants a certain amount and the buyer wants [to pay] less, it is permitted to another to buy it. It is forbidden to encroach on a neighbor’s business territory, in rental of houses and so on from a non-Jew.
There is nothing binding in our case where the deal was negotiated, but neither property nor money exchanged hands.
We rule that Torahitically, it’s the the exchange of money that causes the transition of ownership from buyer to seller. However Rabbinically, the rabbis use their power to redistribute wealth to instead make the sale of objects not used for a mitzvah final when the object being bought is exchanged. (E.g. Say someone sold some fine china, received the money, but didn’t ship the object yet. Then the china breaks. Torahitically, it’s the buyer’s loss. Rabinically this was shifted to be the seller’s loss — he either has to replace the item or return the money. A more frequent but more complicated example is if the deal is done in a hyperinflationary environment, and therefore the value of the money can measurably change before delivery.) Real estate, and objects used for a mitzvah (e.g. buying a share of someone’s chanukah candles immediately before the lighting) are transfered where the money is. Here, neither occured.
When Moshe goes out to the slaveyards of Egypt, sees one Jew striking another, “and he says to the rasha, ‘Why are you striking your neighbor?” (Shemos 2:13) A Jew who strikes another is summarily branded a rasha.
The person didn’t cheat financially. And yet, their word has to be binding (regardless of whom he gave it to). Someone who breaks their word is classed with someone who hits another — both are labeled “rasha“.