Halakha Mazmur 15:2: Kitzur Shulchan Arukh

A man has the moral obligation to keep his word even though he gave no deposit as yet, nor did he mark the article, nor was the transaction completed. [If the parties] agreed on the price neither [the buyer nor the seller] may back out. Whoever retracts whether buyer or seller is guilty of acting in bad faith, and the spirit of the Sages do not look kindly on him, for a Jew should keep his word, as it is written, "The remnant of Israel shall not commit injustice nor speak lies."¹⁰Zephaniah 3: 13. [In addition], a God fearing man should carry out even his unspoken decisions.¹¹Literally, the thoughts of his heart. If he decided to sell a certain article at a certain price and the buyer, not knowing [the seller's] innermost thoughts, offered him more, he should only accept from him the amount he had [originally] decided to charge, fulfilling the verse that states: "And speaks the truth within his heart."¹²Psalms 15: 2. The same rule applies to a buyer; if he decided to buy an article at a certain price, he may not change his mind. This rule applies to all matters in relationships between man and his neighbor. If he decided to do a certain favor [to someone] and is able to do it, he should fulfill his intention. But regarding his personal needs, if it does not involve the performance of a mitzvah, he need not fulfill even [intentions] expressed with his lips.