What is the legal effect when a master tells his Muddabbar (manumitted slave by will) that he will be freed if he pays a specific amount to the heirs?

Primary text

If a master states to his Muddabbar, 'If you pay such-and-such to my heirs, then you are free,' this constitutes a revocation of the manumission (Tadbir). If a jurist holds that the master has the right to revoke the Tadbir by statement, then the Tadbir is nullified in this instance. If the jurist holds that the master does not have the right to revoke, then the statement has no effect.