Moses accepts the deal — warily. Targum Pseudo-Jonathan on Exodus 8:25 preserves the careful language: I will go forth from thee, and pray before the Lord to remove the swarm of wild beasts from Pharoh, and from his servants, and from his people, tomorrow; only let not Pharoh add to deceive, in not releasing the people to offer the festal sacrifices before the Lord.
Two things happen in this verse. First, Moses agrees to pray. He will do what the king has asked, even though he knows the king is unreliable. Prayer is not withheld because the petitioner is a liar. The prophet will intercede anyway. That itself is a lesson: Jewish prayer does not require the object of prayer to have earned it.
Second, Moses names the pattern. Let not Pharoh add to deceive. The Aramaic verb l'hosofa l'shakra — to add to deceive — captures something subtle. Pharaoh has already lied once. The concern is not a fresh lie; it is another lie layered on top of the old one. Moses is watching the compounding interest of untrustworthiness.
The meturgeman is teaching a pastoral skill. When you pray for someone who has betrayed their word, you can do two things at once: fulfill the request, and state the expectation. Moses does both. He prays, and he warns.
The takeaway: generosity and clarity can coexist. You can rescue a king from wild beasts and still tell him to his face that you know he'll try to cheat you again.