Customer Contract: Team Security and Restrict Business Units

Summary:
How do I use Contract team security and at the same time also restrict the possible values of Business Unit during Contract Creation?

Content (please ensure you mask any confidential information):

I assigned Customer Contract Team Member abstract role to the user and assigned Resource Team, because user is not allowed Customer Contract Administrator job role (user not allowed Contract setup access), and it is also possible that a contract can be accessed by users from different BUs but working in the same team.

However, I also want to restrict what Business Unit is displayed while creating contract. For example, User A can only create contract under Business Unit A and Legal Entity A. But User A can allow User B from Business Unit B to access the contract due to cross-BU collaboration (common scenario for Sales).