Exodus 3:20
And I will stretch out my hand, and smite Egypt with all my wonders which I will do in the midst thereof: and after that he will let you go. And I will stretch out my hand, and smite Egypt with all my wonders which I will do in the midst thereof: and after that he will let you go.
God told Moses beforehand that Pharaoh would refuse to let Israel go, and that refusal would not take Him by surprise. Moses was told not to let his courage fail when that refusal came, because God would make it the occasion to display signs and wonders before both the Egyptians and Israel. Deliverance would not come through warfare with Moses as general at the head of Israel, as Moses himself preferred, but through God's own mighty power.
Moses would go not as a military leader but as a shepherd with only a rod, which God made a sign of His power, so that human pride would be humbled. God's plan in this was to magnify Himself before His people and to teach both Israel and the Egyptians that there is a living God who has power to save and to destroy. The glory for the deliverance was therefore to be ascribed to God alone. Only after God stretches out His hand and strikes Egypt with His wonders in its midst will Pharaoh let the people go.
That display of power would reveal God's power before oppressors and oppressed alike, and would so terrify the oppressors that they would grant the Israelites' requests for valuable goods. The Israelites' claim to those goods was just payment for years of labor unjustly taken from them.