פירוש על קידושין 31:12

A baraita teaches that a slave goes free either by money, money’s equivalent or a document. The first two are easy to understand and are anchored in the verse. But what about the document? What kind of a document is this? If it is just an IOU, then it is equivalent to money. And seemingly it should not be a document of manumission, because we would think that a slave does not really need a document of manumission. It should be enough that the master simply tells him to go free.
This leads Rava to say that the slave actually belongs bodily to his master and cannot go free without a document of manumission. An oral declaration is not enough.