Is it permissible to issue an invoice for the full original amount—without mentioning the discount—to a buyer for whom the price of the medication was reduced, knowing that he might submit it to an insurance company to reclaim the full amount?

The questioner is obligated to write the invoice with the actual price he received, because this is the truth that conforms to reality, and because the prevalent practice of someone who makes such a request is that he will take the difference for himself. Furthermore, the purpose of the insurance contract is not profit-making, but rather for the company to bear what was actually paid. Therefore, he should seek Allah's forgiveness for what he has done, regret it, and resolve not to return to it.