The module forces to select a cashier before switching to payment screen. Once user clicks on "Payment", custom popup is shown and user can select a cashier by scanning cashier's barcode or by clicking one from the list.
Demo: http://runbot.it-projects.info/demo/pos-addons/10.0
HTML Description: https://apps.odoo.com/apps/modules/10.0/pos_cashier_select/
Usage instructions: doc/index.rst
Changelog: doc/changelog.rst
Tested on Odoo 10.0 ffba5c688ff74a0630f9f70be1d7760a43a7deba