The Master's Forgiveness?

In Jesus' parable about forgiveness, concerning the 10,000 talents and the 100 talents, I just noticed that the master of the slaves first forgives the slave who owed him 10,000 talents, but after that slave's misbehavior he takes back that forgiveness.  If this practice was common, or at least acceptable, in that era, then I would think that if someone had been forgiven debt, they would never sleep well because they would know that at any moment the debt might be reimposed.

This parable is supposed to be, to some degree, a metaphor for how God forgives us. Does the effectiveness of the metaphor extend to the Master's ability or likelihood to un-forgive the debt? Surely not, but what are we supposed to do with this little detail of the parable?