Gaius, On the Lex Julia et Papia, Book XII. Where the same property was bequeathed to Titius and myself, and the testator died on the very day that the legacy began to vest, and he appointed his heir, and I reject the legacy, either on my own account, or as the heir to the estate, I see that the opinion generally prevails that the legacy is partially extinguished. 1Ad Dig. 31,55,1Windscheid: Lehrbuch des Pandektenrechts, 7. Aufl. 1891, Bd. III, § 531, Note 11.Where a person has been appointed heir who cannot receive any of the estate, or only a portion of the same, and he leaves it to a slave belonging to the estate, in the discussion of his capacity to do this it must be determined whether the person of the heir or that of the deceased should be taken into consideration, or whether neither should be. It was settled after many conflicting decisions that, for the reason that there is no master with reference to whose person the question of capacity could arise, the legacy will be acquired by the estate without any impediment whatever; and, on this account, it will certainly belong to him who afterwards becomes the heir, in proportion to the share of the estate which he is entitled to receive, and the remaining portion shall go to those who are called by law to the succession.

Gaius, On the Lex Julia et Papia, Book XII. The vendor can manumit a slave whom he has agreed to sell, and the promisor one whom he has contracted to deliver.