It is late in the game for Pharaoh. Mitzrayim has just endured the penultimate plague: Dark. Pharaoh now knows he has little time left: It is, for him, the bottom of the ninth.
He summons Moshe, as he has done so many times before, and for the first time conducts an earnest negotiation. The king of Egypt now concedes the demand Moshe had made earlier -- everyone may go, even the women and children. Only, says the Pharaoh, you must leave your cattle behind. Moshe declines the offer, and ups the ante. Not only are we going to take our cattle with us, he insists, but you must supplement the herd with some of your own.