What is the entitlement when a condition for a reward for returning a slave is partially fulfilled based on location?

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If someone states, 'Whoever returns my slave from such-and-such a town shall have one dinar,' and a person returns the slave from halfway to that town, they deserve half the reward because they completed half the required action. This ruling applies by analogy to cases where the condition is partially met regarding the object itself; if the condition is, 'Whoever returns my two slaves shall have one dinar,' and someone returns only one of them, they deserve half a dinar because they returned half of the slaves.