What is the ruling when a master promises a slave freedom if he gives him another slave, and the provided slave turns out to be free or rightfully claimed by another?

Primary text

If the master says to his slave, 'If you give me a slave, you are free,' and the slave provides a slave who turns out to be free or rightfully claimed by another, the manumission does not take effect. This is because the condition implied giving ownership of a valid slave, which was not accomplished.