Qitzur Shulchan Arukh – 62:17
Similarly, one who said to his friend that he would give him some small present, and that one depended on him certainly giving him, if he changed his mind and didn’t give him, he is from among the untrustworthy. However, with respect to a large present it isn’t a lack of trustworthiness, because he [the typical recipient] would not rely on such a statement [to necessarily receive it].
In any event [even where the promise is of something large], at the time he said he would give him, he needs to have finally decided, and not ready to change [his mind]. Because to say one thing aloud and another in his heart is forbidden from the Torah. As it is said (Lev. 19:36): “A just ephah and a just hin you shall have.” What does it come to teach with “a just hןn” ? after all a hןn is [a unit of measure] just like an ephah. Rather, (read it as) your “hein” (“yes” in Aramaic, and therefore plausibly a rare synonym for “yes” – “kein” in other Semitic languages like Hebrew) and your “no” should be just (reliable). (Baba Metzia 49A)
All the previous applies (only to gifts promised) to a rich person, but whatever one said (one would give) to a poor person, whether a small present or a large present, cannot go back on this by (halachic) law, because it is considered like a vow, and even if he only decided in his heart to give, he must keep what he thought.
With everything we said before about speech, all the more so when you say something someone else came to count on.