In Roman Law, Lex Iunia Norbana was law made to regulate "praetoric manumissions". Praetors gave protections to slaves freed by manumissio inter amicos and manumissio per epistulam, which were not as formal as the civil law states. By this law, slaves that were freed by those types of manumissions did not become Roman citizens, but rather so-called Latini iuniani, who lived as free men but died as slaves (this way they could not have free successors). When they died their property belonged to their patron.
Justinian made these manumissions a law, so slaves freed this way would stay free.