Ramban on Exodus
ONLY LET YOUR FLOCKS AND YOUR HERDS BE STAYED. Since the Israelites were keepers of cattle,63Genesis 46:32. and all their wealth and belongings consisted of cattle, Pharaoh thought that they would not leave all their possessions and flee from the country. Even if they were to flee, he would be left with their great wealth, as they were very rich in cattle.
Or HaChaim on Exodus
ויקרא פרעה, Pharaoh called, etc. This occurred after the plague had run its course; previously the Egyptians had been incommunicado. Pharaoh expressed his anger over that fact by adopting a severe tone when addressing Moses in verse 28 when Moses had been adamant that even the livestock would accompany them into the desert. He would not have dared threaten Moses unless the plague had already run its course without Moses having prayed for it to be lifted.