Firstly a sub does not belong in a M/s relationship. The mindset of a sub is different than that of a slave.
Those who enter into an M/s relationship usually do so after much discussion of hard and soft limits, setting boundaries, building trust and respect.
There should be no safe words in a M/s relationship, they are for D/s relationships.
The slave becomes the property of the Master, to be used or discarded as the Master sees fit. The slave is entirely dependant upon the Master and it is a function of that role to look after the well being of the slave.
The scenario put forth here does not work for it mixes dynamics from two different lifestyles, two different mindsets.
A sub can safeword and a Dom sould respect that, not so with a Master and slave..