What is the ruling on manumission when one promises to free every slave they subsequently own?

Primary text

Malik ruled that if one specifies a particular slave, manumission occurs upon ownership, but if one says, 'Every slave I own shall be free,' it is invalid. The latter opinion is considered sounder, as it involves the suspension of divorce or manumission before ownership, akin to conditionally divorcing or freeing a stranger contingent on her entering a specific house, which would not take effect even if the marriage and entry occur later.