All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A cashier in a company is considered as a private worker for the owners of the company. So, he is considered as a trustee and the rulings of a trustee apply to him.
As a principal any excess in cash is for the owners so, he/she has to pay it to them and he/she can not take it except with their permission. Ali Hydar Al-Hanafi said: 'If one gives one's goods to a broker to be sold by a given price, if he sells them by a higher price then the extra amount is also for the owner of the goods. The broker does not get except his wages.'
Therefore, it is forbidden for the cashier to take the excess amount, she has to give it to the owners and she is only entitled for her wages. However, she can take the extra amount with the permission of the owners.
Based on the above details, she has to give what she had taken back to the owners.
For the conditions of a woman's work read Fatwa 83953.
Allaah Knows best.