Thoughts of the heart – One who decided in his mind to do a certain deal or favor:[1]

Once someone decides[2] even in his mind to buy a certain item, or agree to a certain matter, then it is proper for every G-d fearing person to fulfill it.[3] Thus, if one resolved in his mind and heart to sell a certain item for a certain sum and the potential buyer was unaware of this resolved sum and offered a higher price for the item, then he should only take from him the amount that he originally resolved in his heart to charge for the product.[4] Likewise, if a buyer has resolved in his heart to buy a product for a certain sum, then he should not go back on his hearts resolve [and pay less for the product, even if he is offered the product for less than his initial resolve to pay].[5] (The same applies for all other matters of the like, between man and his fellow, that he should fulfill the words of his heart if he resolved to do them, such as to do a certain favor for his friend, and he has the ability to do so [then he should fulfill his resolve].)[6]

Personal matters versus matters between man and his fellow:[7] The above requirement for one to keep his word only applies regarding matters that are between man and his friend, however, matters that are personal, between him and his personal needs, then there is no need to fulfill them, if it does not involve any aspect of a Mitzvah, even if he expressed that he will do so.

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[1] See Admur O.C. 156:2; C.M. Dinei Mechira Umatana Halacha 1; See Encyclopedia Talmudit Erech “Michsurei Emuna” Vol. 44 pp. 300-301

[2] However, if he did not make any resolve in his mind and the thought simply passed through his mind, then there is no need even from a perspective of piety to fulfill this passing thought. [Imrei Yaakov on Admur ibid 1:6 Shaar Hatziyon 31]

[3] Admur 156:2 “Even the thoughts of one’s heart that one has decided in his mind, it is proper for every individual who has fear of heaven to fulfill it.”; Admur C.M. Dinei Mechira Umatana Halacha 1 “A God-fearing man is to fulfill even the thoughts of his heart.”; Bava Basra 88a; Makos 24a; Hagahos Ashri 21; See Imrei Yaakov on Admur ibid Biurim “Sheim Chashav Vegamar Belibo”

[4] Admur C.M. ibid; Rabbeinu Gershom  and Rashbam Bava Basra ibid; Rashi Makos 24a in name of Sheilasos Parshas Vayechi 36

The reason: As the verse [Tehillim 15:2] states “Vedover Emes Bilivavo/and a person who speaks truth in his heart.” [Admur O.C. ibid and C.M. ibid; Bava Basra ibid]

[5] Admur C.M. ibid; Hagahos Ashri Bava Basra 5:21; Bava Basra ibid

[6] Admur C.M. ibid in parentheses

[7] Admur C.M. Dinei Mechira Umatana Halacha 1 in parentheses regarding a resolve though and even speech “However his own personal needs, so long as they do not contain any aspect of a Mitzvah, he does not have to fulfill even if he explicitly said that he will do so””; Admur 156:2 regarding thought