Praise be to Allah.
In the answer to question no. 176034, we noted that it is permissible for the one who buys something using a Visa card to accept points, and that is like a reduction in the price.
In the case mentioned in the question, the employee is trusted to disclose the price for which he bought the tickets, and if he gets a reduction or points that can be redeemed for cash, he is obliged to inform his boss about that. Then if the boss gives him permission to keep it, there is nothing wrong with that, otherwise the boss has the right not to pay him more than the actual price that was paid in the end.
The fact that the points or the reduction in price resulted from using the credit card, which belongs to the employee, does not affect the ruling on this issue, such as if the employee obtained points by making a lot of purchases, or he got a reduction by buying from a relative and the like. In all cases, he must disclose the actual price that he paid.
If the employee redeems these points for cash, then that amount should be deducted from the price of the ticket, and the price should be worked out on that basis.
And Allah knows best.