scratch someone's back
scratch (one's) back
To provide one with a favor; to do something generous, helpful, or beneficial for someone. Usually implies that the favor is expected or hoped to be returned in the future. The president has been accused of scratching the oil industry's backs with a series of executive actions that put them in a position to make a lot more money. I've been scratching his back for years now—getting him a job, helping him move into his apartment, lending him money to buy furniture. The least he could do is help me clean out my house before I move. Just get me the information on the senator I need, and I'll make sure it's worth your while—you scratch my back and I'll scratch yours, OK?
scratch someone's back
Do someone a favor in hopes that a favor will be returned. For example, I don't mind driving this time-she's scratched my back plenty of times. It also is put as you scratch my back and I'll scratch yours, as in If you do the laundry I'll do the cooking-you scratch my back and I'll scratch yours. This idiom was first recorded in 1704.