Sounds like fruad to me.
1. Do you a contract of employment?
2. Is the commission that you were paid mentioned in this?
3. If you did not have a formal contract of emplyoment, was theer a
verbal agreement that a certain % of commission would be paid to
you?
If not, looks like the manager was paying you a comission without
proper authority.
As Laurenboi says, how do you balance the cash if you don't punch
the register? Who banks the cash and how is it tallied to the
sales?
If there is cash or stocks are missing or unaccounted YOU could be
blamed and charged with theft.
Don't do it. If manager insists that you do it, get instructione IN
WRITING. You can then show this as evidence (to the big bosses,
police or courts) that you were only doing what you were instructed
to do.