Asset (15-16): Paul hinted at his desire to have Onesimus serve him in Rome (13-14), but he doesn't demand Philemon to grant him this. In His providence, God may bring about a different outcome. Onesimus may have run away so that Philemon may ultimately have him as a beloved brother. With a Christian slave, he wouldn't just have a man useful for service in the flesh but for service in the Lord. Onesimus is going to be a spiritual asset!
Partnership (17): Paul then makes his request known. Philemon is to treat Onesimus as he would treat the Apostle, as a partner in the faith. Paul is sure they share in this partnership (another word for "fellowship") of believers (1:6). Besides this, Onesimus is now also a part of this fellowship (1 John 1:7). Believers' partnership with one another is real and practical, not theoretical.
Repayment (17-19): Paul also models this partnership himself. He will fully assume whatever debt Onesimus owed Philemon. And Paul is dead serious. He writes this promise of repayment with his own hand! Paul saw his material possessions rightly (Acts 2:45; 20:35) and loved people with his heart and his wallet! Philemon received such a love in the past. Now, he is to pay it forward.
 "Forever [ai-ōn-i-os (αἰώνιος)]" points to eternity. Spiritual brotherhood would remain unto eternal life.
 If Paul expected manumission, he would have incurred the payment for his freedom, not his debt. "No longer as [i.e., in the character of] a slave" points to Onesimus' greater value than a mere slave ("more than a slave"). Christian masters were not required to free their slaves, but to treat them well (cf. Eph. 6:9; Col. 4:1).
 Onesimus is expected to serve his believing master even more heartily as a Christian slave (cf. 1 Tim. 6:2).
 "In the flesh and in the Lord" modifies the main verb "have him back." Given the juxtaposition of slave and brother in this sentence, "in the flesh" best describes service as a slave and "in the Lord" Christian ministry.
 "If then you regard me" is a first class condition with the sense of "Since." Paul is sure of this relationship.
 Letters were often written with an amanuensis (Rom. 16:22; 1 Pet. 5:12). Paul habitually writes his own greetings (Col. 4:18; 2 Thess. 3:17). This is the only time when he inserts his own writing for a non-greeting.