Philemon 10-11 (NIV)
I appeal to you for my son Onesimus, who became my son while I was in chains. Formerly he was useless to you, but now he has become useful both to you and to me.
Philemon 18 (NIV)
If he has done you any wrong or owes you anything, charge it to me.
Onesimus was a slave of Philemon who apparently ran away and probably stole some valuables in the process. Paul met Onesimus and led him to the Lord and is now sending him back to Philemon a changed man.
In this short letter, Paul illustrates for us the principle of reckoning by which God justifies us. It is an accounting transaction that Paul acts out by charging to his own account any debts Onesimus owed to Philemon. The Bible speaks of sin as a debt--owed to God. Jesus took our sins upon himself effectively removing the barrier between us and God.
But that is only the debit side of the accounting entry. Not only did Jesus transfer our debt to himself, (here is the glorious part) He transferred his righteousness to our account.
2 Corinthians 5:21 (NKJV)
For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him.