Often times we are moved by a tzedaka cause when hearing about it and we decide that we will give a certain sum to the cause. Later as we cool off we may decide that we don't want to give so generously. Are we still obligated to give the original sum or not?
The Shevet HaKehosi (1:257) brings the Rema (YD 258) who says that it is a machlokes whether a mere mental decision of giving tzedaka obligates us or not. The Rema concludes that we should be machmir and consider the thought a done deal.
However, says the Shevet HaKehosi, there are decisions and there are decisions. In this case, he brings from Rav Yaakov Emden that only if your decision was final and without any hesitation are you obligated. If it was just an idea to give, then it is not binding and you may change your mind.
He also brings the Mogen Avrohom (562:11) who says regarding someone who decides to be Mikabel Taanis in his mind, that it is only binding if he commits "B'Libo" in his heart. If he only thinks "B'Daato" in his mind, then the Kabala is not binding.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.