Musar zu Schemot 5:6

You should not think that Pharaoh's increasing of the Jewish people's workload (Exodus 5,6-9), was merely his own idea. On the contrary, it represented the will of G–d. He had sent Moses to Pharaoh in order to produce this result. This is why two expressions are used when G–d enlists Moses' services. A) לכה, B) ואשלחך (Exodus 3,10). The first expression usually refers to an undertaking due to one's own initiative, whereas the second word אשלחך refers to the carrying out of a mission at someone else's bidding. The Ari zal states that G–d sent two missions. One mission had as its purpose the redemption of Israel, i.e. ואשלחך. The other had as its objective the increased burdens imposed by Pharaoh on the Jewish people, i.e. something apparently painful for the Jewish people. Since G–d never describes a mission undertaken at His behest as לרעה, meant to have negative effects, it was couched in language that normally describes one's own initiative, i.e. לכה-לך נא (go please).