Is the employee in a retail store obliged to deposit the profit generated from selling goods not available in the store—and provided from another source—with the store as a benefit, or is it permissible for him to take it personally?

The summary of the answer:

The worker must commit to selling what the shop owner has authorized him to sell, and it is not permissible for him to sell anything in the shop for his personal benefit without the shop owner's permission, whether the item sold is owned by the shop owner or by the worker himself; this is because the benefits of a private worker are owned by the employer. If the shop owner authorizes the worker to buy and sell goods that are not available, there is no harm, and the profit belongs to the shop owner unless they agree on a specific division. However, if the worker buys with the shop owner's money without his permission, the profit belongs to the shop owner. And if he buys with his own money without causing harm to the shop owner, the profit belongs to the worker, and he bears the sin of transgression.