More detailed quote:
The same holds true for the maidservant who is under the yoke of slavery; she too is in another's power, that is, her master's. But if she is a believer, let her regard herself as belonging to God, not to her master, and let her keep her body pure for the Lord who bought her with his own blood. [...] But if her master is an unbeliever, let the believing maidservant bear up, knowing that she is serving the Lord Christ. Let her not maltreat her flesh but rather adorn it chastely, keeping it pure and undefiled for the Lord. For it is not permitted her to fornicate or to surrender her body to anyone but her own husband. But if she is forced by her master, let her pray to be given strength to endure and not to sin against the Lord. For the body is a temple of the Holy Spirit and must be kept in holiness.
Upon reading better I agree it's more likely he was referring with 'endure' to persist in refusal than to let her master have his way with a sinful plan. Under Torah law though the master could have married her still himself but any believing master would never be allowed to force his slaves to commit sin - including prostitution.
PS: later theologians do seem to provide this exoneration in case of force/coercion - e.g Augustine (The city of God - +/- 420 AD):
And this seems a rhetorical question.