If one states that they will do something to another person (e.g "I'll give you my favorite necklace"), and then that person no longer wants to fulfil the "promise" due to unforeseen circumstances, is that forbidden? What if the statement was only written and not verbally uttered?
Similarly, if a person promised to give a colleague a gift and failed to do so, he is considered to be faithless. When does the above apply? With regard to a small gift, because the recipient will depend on the promise that he was given. With regard to a large gift, by contrast, the giver is not considered to be faithless if he retracts, because the recipient does not believe that he will give him these articles until he transfers ownership through a formal kinyan.
The Shulchan Aruch codifies this in CM 204:8
dinonline (here) writes
The precise definition of which gift is considered small and which is large depends on the circumstances of the giver: For a poor individual a gift of $50 may be large, whereas for a millionaire a gift of $1,000 can be considered small.
See there at length, particularly the end section "Keeping One’s Word in Transactions".
As always, treat the above as sources, but don't make real-life decisions without speaking to your rav. There might be mitigating factors one way or another that will impact the final ruling