What are the legal consequences of manumission upon payment in an invalid Mukatabah contract?

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In an invalid Mukatabah where manumission occurs upon payment, the manumitted individual is not required to pay the value of himself, nor can he reclaim what he paid to his master. This is supported by Abu Bakr and aligns with the apparent position of Ahmad. The basis for this is that the contract is a Mukatabah intended for exchange where manumission is achieved by payment, thus it does not necessitate full nullification like a void contract. Furthermore, what the master received is from the earnings of the slave, whose earnings he did not own, so return is not incumbent upon the master. The slave is freed by the condition, so his value is not due.