I'll (have to) let you go
A phrase said to facilitate the end of an interaction with someone, often over the phone. It sounds pretty busy there, so I'll let you go. Call me tomorrow, OK? Sure, I'll let you go, but let's make a lunch date soon. Not that running into you in the grocery store isn't lovely!
I'll (have to) let you go.
Fig. It is time to end this phone conversation. Well, I'll let you go. It's getting late. I have to go to work early tomorrow, so I'll let you go.