If the market value of the product has changed, or other unexpected changes occurred:[1]

Some Poskim[2] rule that it is forbidden [due to Michsurei Emuna] for either party to retract from the sale price even if the market value has changed. Hence, if the value of the product went up in price, the seller may not demand a higher price, and if the value of the product went down in price, the buyer may not demand it to be given to him for the lower price, and if either party does so then they are considered untrustworthy. Other Poskim[3], however rule that if one of the parties retracts from the sale agreement due to an increase or decrease in the market value of the item, then he is not considered untrustworthy, and hence even initially one may choose to do so. [The same applies regarding any other unexpected change on the ground that may make the person now retract from the sale agreement.[4]] Practically, the main ruling follows the former opinion[5], [although some conclude that it is a mere Midas Chassidus to be stringent[6], and some write that the custom today is to be lenient in this matter[7], and by a large unexpected change, then one may be lenient according to all[8]].

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[1] See Rama C.M. 204:11; Smeh C.M. 204:12; Shach 204:8 in name of Bach 204:11 leaves this matter in question; Aruch Hashulchan 204:8; Encyclopedia Talmudit ibid pp. 312-314; Alon Hamishpat Vol. 51 for article by Rav Yosef Fleishman; Imrei Yaakov on Admur ibid 1:5 Shaareiy Tziyon 27-29

[2] 2^nd opinion of Rama ibid; Darkei Moshe 204:11; Smeh C.M. 204:12 in opinion of Rambam Mechira 7:8 and Setimas Michaber [and so would likewise follow to be Setimas Admur]; Nimukei Yosef on Rif Bava Metzia 49a in name of Raavad and Rashba; Magiid Mishneh Mechira 7:8; Ramban in Milchamos on Baal Hamaor ibid; Beis Yosef 204 in name of Tosafus; Rabbeinu Yerucham Nesiv 9:4; Poskim in Encyclopedia Talmudit ibid footnotes 255-265; Poskim in Alon Hamishpat ibid footnote 2 and 6-7 and that so rule majority of Rishonim

[3] 1^st opinion of Rama ibid; Smeh C.M. 204:12 in opinion of Rosh Bava Metzia 4:14 and Tur 204:11 in name of Razah in Baal Hamaor Bava Metzia 29; Talmidei Harashba in name of Ittur; Yerushalmi Bava Metzia 4:3; Poskim in Encyclopedia Talmudit ibid footnotes 248-254; Poskim in Alon Hamishpat ibid footnote 1 and 8-9

[4] See Chasam Sofer C.M.  102; Encyclopedia Talmudit ibid p. 314 footnotes 266-269; Alon Hamishpat ibid6-10  footnotes 12-15; ; See Imrei Yaakov on Admur ibid Likkutim 1:1 Mekoros 1-2

[5] Rama ibid “Vichein Nireh Ikkur” [like 2^nd opinion in Rama, which is our 1^st opinion brought above]

[6] See Aruch Hashulchan ibid who concludes that it is only Midas Chasidus to be stringent, and not a requirement; Alon Hamishpat ibid footnote 10

[7] Shevet Halevi 4:206 based on Chasam Sofer ibid; Rav M.M. Shpern in Hayashar Vehatov 9:38 and Rav Sariel Rosenberg in Hayashar Vehatov 8:52;  Poskim in Alon Hamishpat ibid footnote 11

[8] See Alon Hamishpat ibid