Is the pharmacy worker sinful if he gives the customer an invoice for the actual price of the medication without the discount, knowing that the customer will be reimbursed by his employer, and there is a price difference due to the discount?

Only the actual price paid should be written on the invoice, so that the buyer does not deceive their employer and take from them more than what they paid, making the invoice writer an accomplice in sin. It is permissible to write the original price, the discount percentage, and the price paid to clarify this on the invoice and avoid assisting deceivers.