What stops a cashier ringing up a sale and pocketing the cash?

Several small things that add up. Every sale carries the name of whoever was signed in. Giving money back, cancelling a sale that has already printed and taking money out of the drawer all need a manager and a written reason. At closing the POS asks what was counted and shows it against what should be there, and cancellations after printing appear on the shifts report where you can see them.