The prohibition against making a promise or agreement without intent to fulfill it:[1]

Although, as will be explained, there exist various scenarios in which one retains the legal right to retract from a promise or agreement, and at times doing so is not forbidden and does not even consider one untrustworthy [i.e. a promise to give a large gift, as will be explained], nonetheless there exists a Biblical[2] prohibition against one making a promise or agreement without intent to follow through at the time the one is making it.[3] [Meaning, although changing one’s mind at a later point is not considered outright lying, and one retains a legal, and at times even a moral, right to do so based on new developments or points that he is now taking into account, nonetheless, at the time that he makes the promise or agreement he must have in mind to fulfill it, and if he changes his mind later, then he changes his mind. However, to make a false promise or agreement which he has no intent to fulfill even at the time that he is saying it, is Biblically forbidden. Thus, in all scenarios to be explained that one is allowed to change his mind, or that changing his mind does not transgress a Biblical prohibition, this is only if he intended to follow through with his promise and agreement at the time that it was made.]

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[1] Admur C.M. Dinei Mechira Umatana Halacha 2; Abayey Bava Metzia 49a; Rebbe Yaakov Ben Zavdi in Yerushalmi Shevi’is 10:4; Rosh Bava Metiza 4:12; Rif Bava Metzia 29b; Nimukei Yosef Bava Metzia 49a and Hamaor Hagadol in explanation of Rif ibid that so applies even according to Rebbe Yochanon [However, see Minchas Pitim 204:11 that this applies only according to Rav]; Yerushalmi end of Shevi’is; Rashi Bava Metzia 49a; Shaareiy Teshuvah of Rabbeinu Yona 182; Minchas Pitim ibid in name of Ramban in Milchamos, Shita Mekubetzes Kesubos 86a; Poskim in Encyclopedia Talmudit ibid footnotes 62-66

Other opinions: Some Poskim imply that making a statement without intent to follow through with it at the time that one says the statement, is not under any Biblical prohibition. [Implication of Admur O.C. 156:2, and all Poskim who hold of this position, who interprets this verse to refer to the concept that one should always try to follow through with his promises, which is only a matter of Michisurei Emuna, and is not Biblical]

[2] Admur ibid based on Milchamos on Rif ibid and Rosh ibid; Implication of Nimukei Yosef Bava Metzia 49a “Over Al Esei Deoraiysa”; Imrei Yaakov on Admur ibid 1:9 Shaar Hatziyon 36; See Likkutei Sichos 32:157 footnote 24

[3] The reason: As the verse [Vayikra 19:36] which discusses the obligation for one to have honest and justified weights in commerce states, “Eifas Tzedek Vehin Tzedek Yihyeh.” Now, why does the verse repeat the term weights using the word “Hin,” as was this term not already included in the word “Eifas?” Why the redundancy? Rather, it is coming to teach us that one’s yes and no should be justified. [Admur ibid; Abayey Bava Metzia 49a; Rif 29b; Nimukei Yosef Bava Metzia 49a in explanation of Rif ibid that so applies even according to Rebbe Yochanon; Rambam Deios 2:6] See Rashi ibid that the Hin is 12 Lugin while the Eifa is three Sean which is 72 Lugin

Other opinion of Admur: Elsewhere, Admur O.C. 156:2 interprets this verse to refer to the concept that one should always try to follow through with his promises. In his words: “One is to beware to deal honestly in business and not change his word as the verse [Kedoshim 19:36] states, “Vehin Tzedek Yihyeh Lach,” which is interpreted to mean that one’s “yes” and “no” should be justified. This means that when a person speaks and agrees to something with a yes or no, then one must try to fulfill his word and align them with his actions.” This follows the opinion who rules that the above verse is a mere Asmachta to the concept of Michisurei Emuna, and is not of Biblical status. [Braisa in Bava Metzia 49a in understanding of question of Gemara; Smeh 204:12; Rav Ovadia Bartenura and Tosafus Yom Tov and Pirush Hamish ayos of Rambam on Shevi’is 10]