Hi Tomas,
There is no direct way of linking the accounts to the BU's. From a security point of view it is owned by a user who belongs to the BU and this is how CRM controls the access.
The only option what you are left with (using standard options) is to create 'Teams' by Sales Manager per Territory and share (read, write, append, assign, delete, share) the records individually where you will have some amount of control.