Why did God send Hagar back to Abraham and Sarah before Ishmael was born?
God sent Hagar back to Abraham and Sarah before Ishmael was born so that the later events would become a New Testament lesson. It parallels how God saved Cornelius to prepare Peter, twelve years later, to defend Paul's gospel to the Gentiles at the Acts 15 council.
Les explains that God sent Hagar back to the tent of Abraham and Sarah even before the child was born, just to have to go through the same thing thirteen years later. Why? So that we could have a New Testament lesson. He then applies this same pattern to Peter and Cornelius. Les teaches that God wanted the house of Cornelius saved, no doubt, but the bigger picture was that God had to prepare Peter for the event in Acts 15, where Peter would come to Paul's defense and remember that God had saved Gentiles by his preaching twelve years earlier. So the Hagar situation was providentially designed to provide a New Testament lesson, just as the Cornelius event was providentially designed to prepare Peter for Acts 15.