Use Case #10. Bi-directional relationships with servers
You have a server with data from your POS to be integrated with srv4pos.com where articles/sales can come both from srv4pos and your server.
A full-featured master-master synchronization is used between srv4pos.com and regular POS as described in case Use Case #5. Regular POS because regular POS has own database. Use the same method for POS server too.