What did God already know when He told Hagar to return to Sarah's tent?
God already knew that thirteen years later He would instruct Hagar and Ishmael to leave and return to the desert, the very place He had brought her from. The command to return to Sarah's tent was temporary, setting the stage for that later departure.
Les explains that Hagar, while pregnant, taunted Sarah until Sarah told Abraham to send her away. God intervened in the desert and told Hagar to go right back to Sarah's tent and stay there. But God already knew that thirteen years later He would tell Hagar and Ishmael to go right back to the desert, the place He had brought her from originally. This fits the larger contrast between Isaac and Ishmael, the flesh and the Spirit, and the casting out of the bondwoman.