if its down then that staff member must account for it or pay for it themselves.
And if it's up? Presumably the staff member gets to keep the excess stock or the monetary value.
No. I know that doesn't really happen. What will happen, if there is over-stock, is the staff member will be under suspicion of diddling customers, even though that diddling will be in favour of the employer and not the staff member themself.
So, once again, and as many have said; if a customer wants hot water and is willing to pay for it then sell it and do what the heck you like with the teabag.