We are learning from Case Studies in Business Ethics by the late Rabbi Dr. Aaron Levine z"l professor of economics at Yeshiva University.
Another scenario where retracting a promise is not considered unethical:
If A were to promise B a certain item as a gift for B's birthday in the future, but before the birthday arrives the price of the item rises sharply and A retracts.
The Talmud brings a similar involving a sales transaction. Buyer B and Seller S commit themselves to a sales transaction but before the transaction reaches the point where either B makes a deposit, or the point where the parties would become otherwise legally bound, the price sharply rises and S wishes to retract.
Talmudic authorities dispute whether S's retraction is considered unethical.
Rabbi Yechiel Michel Epstein (Belarus, 1829-1908) in his authoritative work Aruch Hashulchan rules that S would not be considered untrustworthy in such a case.
Presumably he would rule the same in our birthday case.