The preacher made a casual passing reference to the verse that refers to the word imputation. I think imputation is probably one of the most important concepts in Christianity. In verses 17 and 18 Paul asks Philemon to view Onesimus as if he were Paul himself, to welcome him as he would Paul and to charge any debt Onesimus owed to Paul's account.
This substitution of Onesimus for Paul is the same transaction Jesus accomplished for us. Jesus took our sin like Paul accepted the debt of Onesimus to Philemon, and Jesus gave us his righteousness like Paul asked Philemon to accept Onesimus as he would have welcomed Paul. This simple transaction gets confused with the words which try to explain it.
There is one problem which neither the text nor the preacher explored. Confession and repentance are not addressed; however, I think they are implied. The text does not say whether Onesimus was just visiting Paul or whether he was also a prisoner. In either case, his sorrow over his violation of Philemon's property is implied. For him to be willing to return to Philemon, Onesimus must have felt profound humiliation. For him to have sought and received salvation under Paul's instruction must have also included confession and repentance.
A second chance without confession and repentance is pointless. A second chance is founded on a lesson learned.