Is a bequest valid if it concerns what is owed by a Mukatab (indentured servant) whose contract (Kitabah) is invalid?

Primary text

The bequest of property owed by an indentured servant whose contract is invalid is not valid because the servant possesses nothing in his liability (Dhimmah) under that contract. This ruling is established upon the principle that a disposition cannot concern that which does not legally exist or belong to the testator.