What is the ruling when a testator instructs his heirs to free one slave ('Aatiqu raqabatan 'anni')?

Primary text

When a testator instructs his heirs to free one slave without qualification, only a Muslim slave shall be freed on his behalf. This is because an unqualified instruction from a human being is interpreted in light of the unqualified instruction in the Book of Allah, and since Allah commanded the freeing of a 'neck' (*raqaba*), it only encompasses a Muslim slave.