Onesimus had so endeared himself to Paul that his departure was an extremely painful prospect for the apostle. Paul could have justified keeping the slave with him, but he judged that Onesimus" obligation to return to his owner was more important. Furthermore, Paul did not really have authority over the slave; that rested with his master. If Paul had kept Onesimus with him, Philemon would have felt obligated by his regard for Paul to let his slave stay with the apostle. The service Paul probably had in mind for Onesimus was to proclaim the gospel, not to perform menial prison duties for Paul. [1] Nevertheless, Paul wanted Philemon to respond to his slave freely. [source][source][source]
"The principle of consideration for others here manifested by Paul is a factor of vital importance today for effective Christian leadership. Many are the difficulties which might be avoided if those in places of authority in Christian work would follow Paul"s example in this." [2][source]
"In the eastern part of the Roman Empire [3] during this period, fugitive slaves who sought sanctuary in a household were likely to be given temporary protection by the householder until either a reconciliation with the master had been effected or else the slave had been put up for sale in the market and the resulting price paid to the owner ..." [4][source]