בבא בתרא קנב:
"דברי שכיב מרע ככתובין ומסורין"
We know [קנא.] "דברי שכיב מרע ככתובין ומסורין". This means that although to give a gift or transfer ownership of an item, a קנין is always required, the רבנן were מתקן that this is not necessary for a שכיב מרע. The [קמז:] רשב"ם explains, this was established for fear that the שכ"מ might be anguished at the prospect of his children not fulfilling his last wishes. The רבנן were concerned that this pain could aggravate his already dire medical condition, so to give him peace of mind, all that is required of him is to verbally say to whom he would like to bequeath his estate and it will be halachically binding.שמואל says, if a שכ"מ gives away his possessions but uses a (סודר או שטר) קנין, we must assume that he doesn’t want to take advantage of this תקנת חכמים and consequently his מתנה is NOT effective. This is because, as all people who are very ill, he wants this קנין to be חל only after his death, however, a קנין can’t transfer property after one’s death, because the יורשים have inherited it at this point. Only the special תקנת דרבנן of a שכ"מ can accomplish this. So, if the קנין can’t work and the תקנה doesn’t work, his transfer is void. [ב"ב אות תקנד] ר' אלחנן brings the שו"ת הרמ"א who quotes the psak of the בית יוסף regarding a שכ"מ that gave away his נכסים to צדקה, but reinforced it with a קנין, the ב"י says the יורשים must be מקיים דברי המת. The רמ"א questions this psak from our Gemara that says if the שכ"מ uses aקנין , it is מבטל the תקנת שכ"מ and the צוואה is בטל.ר' אלחנן defends the ב"י and says the דין is a person may say "אי אפשי בתקנת חכמים". This is because since a תקנה is for his benefit, he may opt out if he so desires. Therefore, when we see a שכ"מ using a שטר, we assume he is saying אי אפשי that my verbal instructions should work and I prefer to use a קנין. However, in the case of the ב"י we say אמירתו לגבוה כמסירתו להדיוט. So even if he adds a קנין to this pledge of צדקה, it doesn’t undo the אמירה, so the pledge remains valid. [קלז:] אביי says: "מתנת שכ"מ לא קנה אלא לאחר מיתה". Although we say the words and instructions of a שכ"מ are ככתובין ומסורין, this is only חל after the מיתה. This is how we pasken [טור חו"מ ס' רמ"ח,ד]. However, when the טור [חו"מ ר"נ,א] explains the דינים of שכ"מ he says that after the שכ"מ dies, the מקבל is קונה the property למפרע משעת נתינה. The בית יוסף says קשה לי, because we said above that the מקבל is only קונה after מיתה? The ב"ח answers that the טור means to say that when we say ככתובין ומסורין we mean we view it after מיתה as if it happened now, before his death. לאפוקי saying that we only view his words after his death and then it’s ככתובין ומסורין, that would not work as קנינים can’t be חל after מיתה because the יורשים have inherited it first.