What is the ruling concerning a non-Muslim's bequest stipulating service to a church followed by freedom for a slave?

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If a non-Muslim stipulates in his will that a specific slave must serve a church for five years, after which he is free, and the master dies after the slave served one year, and then the slave converts to Islam, the slave becomes free immediately upon the master's death, because the condition (serving the church) is associated with sin. If the bequest was valid, the slave is due the compensation for the remaining four years of service, as the manumission was contingent upon an exchange that became invalid due to conversion. However, the sounder narration is that the slave is free immediately upon the master's death because the underlying condition constitutes a sin. The opinion suggesting compensation for the remaining service is less likely to be based on the validity of the bequest itself, but rather on the premise that the owner intended to free the slave via a substitute consideration which failed upon conversion, thus necessitating a compensatory payment, similar to a Dhimmi spouse converting and thus owing the dower.