פירוש על קידושין 36:4

The rabbis read the words “bevigdo bah” as referring to the master designating her for himself. Once the master has married her, her father may not sell her to anyone else. But he may, by inference, designate her for another man if the master dies or divorces her. In other words, he can marry her off again. However, in general a father cannot give her daughter away in marriage once she has already been married. Therefore, this must mean that when the master designated her to become a wife for himself, he caused her to be betrothed, not married.