What is the expiation for someone who said "we hear and we obey" regarding a good deed he intended to do but then did not do? And is this similar to an oath?

An oath is only valid when made in the name of Allah or one of His attributes, as stated by the esteemed scholar Khalil al-Maliki in al-Mukhtasar: "An oath is the affirmation of something not yet obligatory, by mentioning the name of Allah or one of His attributes, such as 'by Allah.'" Accordingly, saying "sam'an wa ta'atan" (we hear and obey) is not considered an oath, nor does an oath become binding by it, nor does it necessitate expiation.