Jamieson, Fausset and Brown on Exodus 10:24-26

24-26. Pharaoh called unto Moses, and said, Go ye, serve the Lord--Terrified by the preternatural darkness, the stubborn king relents, and proposes another compromise--the flocks and herds to be left as hostages for their return. But the crisis is approaching, and Moses insists on every iota of his demand. The cattle would be needed for sacrifice--how many or how few could not be known till their arrival at the scene of religious observance. But the emancipation of Israel from Egyptian bondage was to be complete.