(Mishnah): If a deathly ill person wrote over all of his property to his brothers...if he withheld any field for himself, the gift remains even if he recuperates; if he did not withhold anything, the gift is returned to him if he recuperates.

R. Meir: If the document did not specify that he was deathly ill but he said that he was deathly ill at the time and the brothers claim that he was healthy, he must prove that he was deathly ill.

Chachamim: Since they wish to take the property from his possession, they must prove it.

(Gemara) - R. Yirmiyah citing Rav: But if he only withheld movables, the gift is returned, unless he withheld money for himself to buy land, in which case, it is as if he actually withheld land.

If he withheld any sized field, his gift is valid even if he did not recuperate. If he did not withhold any field, his gift is valid only if he recuperates.

This is like that the statement of R. Yochanan citing R. Yanai - they equated the words of a deathly ill person to a healthy person who wrote and gave a document. This is only true when he actually died from that illness.