As we have learned (see daf 126) a man cannot choose to change biblical inheritance laws, but he does have the right and the ability to divide his wealth any way he wants to while he is still alive. The Mishna (126b) teaches that as long as he says that what he is doing is a gift it will work, even if he also says that he means it to be an inheritance. The Gemara on our daf explains that this will work in all cases where the statements are made tokh ke-dei dibbur – when they are said “in the same breath.”
The Rashbam explains that when two statements are made in the same breath, it is as if they were said simultaneously. Therefore, if we can apply both statements, we will do so. If only one of them is a viable statement – like in our case where the two statements refer to a gift and inheritance and only the statement about the gift can be applied, that is the one that we listen to. If the two statements contradict, then we have a difference of opinion as to whether we should listen to the first statement or the second, or perhaps divide the thing in question or leave it in the hands of the person who has possession of it. In any case, if the second statement was one where the person says “I take back my first statement” then that is what we give credence to.
The foundation of this rule is, according to Rabbeinu Tam, of rabbinic origin. The Sages wanted to give someone who was in the midst of negotiations the opportunity to greet his teacher, and then return to his business without affecting it. Other rishonim explain that this law has biblical effect, and is simply based on the idea that until a person has completed his statement he can alter or negate it.