Qitzur Shulchan Arukh – 62:16
It is proper for a person to keep his word. So that even if he didn’t yet pay money, and didn’t mark the object, nor completed the transaction, if they have agreed on the price, neither of them should go back. The one who goes back, whether the buyer or the seller, is considered among the untrustworthy, and the sages’ spirits do not rest well about him.
Because it is proper for a Jew to keep his word, as it is said (Zephaniah 3:13): “The remnant of Israel shall not do iniquity, nor speak lies.” One who is aware of G-d’s Significance (yarei Shamayim) should fulfill even what his heart’s thoughts. So, if he thought and decided in his heart to sell him at a particular price, even though the other did not know his thoughts and offered him more than that amount, he should not take from him except the amount that he had decided on. As to carry out what said (Psalms 15:2): “he … speaks truth in his heart.” Similarly, the buyer who decided in his heart to buy for this-and-this amount, he should not go back on this.
Similarly all things concerning other dealings that are between a person and his fellowman, he should carry out the decisions of his heart. Of he decided in his heart to do a favor, and he was able to do it [he ought to]. However, things for himself, as long as they are not desirable for a mitzvah, he does not need to carry out, even what he spoke.
I wrote on Friday about 62:15:
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. [In 15] 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.
Here we go the final step in the sequence. Speech and even thought matter. Even when nothing at all was done, even where the context is outside of business.
To add a little emphasis…
וַיֹּ֥אמֶר אֱלֹהִ֖ים יְהִ֣י א֑וֹר וַֽיְהִי־אֽוֹר
And Hashem said, “Let there be Light!” and it was light.
– Bereishis 1:3
The Baal Shem Tov points out that it’s not that Hashem’s speech caused the light to appear. Rather, the speech is light. That which we call “light” is actually Hashem saying “יְהִ֣י א֑וֹר”! (The Besh”t then continues to use this idea to explain continuous creation. Words in a book are written once, and then persist. The spoke word lasts only as long as the speaker is speaking. Thus, Speech as a model for Creation implies that G-d is continually creating the world anew, “Who renews, in His Goodness, daily, continually, the Act of Creation.”)
Speech is the actual substratum of the universe. A davar, a thing, is a dibur, a statement.
To break a promise, even one never articulated, is to pick at the very fabric of Creation.