Digestorum libri
Ex libro XXIV
Marcellus, Digest, Book XXIV. If a minor under twenty years of age sells you a slave and delivers him, under the condition that you will manumit him, the transfer is of no effect; even though he may have delivered him with the intention that, when he had reached the age of twenty, you should manumit him; for it makes very little difference if the date of his freedom is deferred, for the law opposes a provision of this kind as not being well founded.
Marcellus, Digest, Book XXIV. A certain man extorted a slave from Titius by violence, and directed him to be free by his will. The slave will not become free, even if the testator died solvent; for otherwise, Titius will be defrauded, as he can bring an action against the heir of the deceased on the ground that the bequest of freedom was void; but if the slave should obtain his freedom, Titius will not be entitled to an action, because the heir will not be held to have gained anything by the fraud of the deceased.