ACCESS DATA FOR AGENCY USERS - HELP!!!

I've added a team member to my agency. He is a sales of my agency (THAT REQUIRES ONLY ASSIGNED DATA) but also need to give him access to all the customers with whom he has the commercial relationship to keep in touch, upsell ecc.

The problem is that in my agency subaccount he needs to have access only to assigned data while in the customers subaccount access to all data but if i set "only assigned data" he has that option to all assigned subaccounts.

I tried to change permission from inside a specific sub account from "only assigned data" to "all data" but the setting changed globally for this user that has now access to all the datas o my agency subaccount.