According to the Mishnah on today’s daf (=page) if someone promises to give his friend a monetary gift, it does not have the same significance as someone who actually owes that amount of money to the person. Therefore, if the individual who claims that he was promised a gift demands that the witnesses to the promise step forward and testify, should the witnesses decline to testify, the halakhot of shevu’at ha-eidut – the laws governing cases where a person refuses to offer testimony, swearing that he does not have information that would be useful in court – would not apply.
Essentially, what the Mishnah is teaching – as is noted in the Talmud Yerushalmi – is that a person who makes such a promise retains the right to renege on his promise as long as he has not given the present. The R”i mi-Gash goes so far as to rule that even if a formal kinyan – a formal agreement made with a symbolic act such as a handshake or a symbolic transfer of some item – had been made, the person who made the promise can still back out, since the kinyan was not made on the actual money, rather it referred to the promise itself.
The Tosafot Yom Tov adds that this ruling is true even in a case where the individual to whom the promise was made was a poor person, which may be considered a case where the promise was a promise of tzedakah – charity. Although the Jewish court could step in and obligate him to keep such a promise, nevertheless they can only do so by means of social coercion (e.g. a ban or excommunication), but they cannot actually make use of their judicial powers to confiscate the money from him and give it to the poor man. This shows that the promise does not create an actual monetary obligation.