Gittin 43 - A woman who is half-slave, half-free

If a master sells his slave only for the right to collect penalty payment in the event the slave is killed by an ox, is the sale valid?

Since there is a law about the rights of a slave whose value is zero, the sale must be invalid, because any slave should have at least this value?

Rebuttal: no, if the slave is mortally ill, there is no penalty for goring.

Result: answer is unknown.

If one engages a half-slave, half-free woman, she is engaged, but if one engages half of a free woman, she is not engaged.