Is it permissible to inform the employer about what the purchasing agent is doing, such as taking a discount for himself and asking for money in exchange for his services, knowing that the employer is kind and trusts him, and I do not have strong evidence against the agent?

These matters are forbidden by Islamic law because they involve cheating, treachery, and bribery. Allah has prohibited them in the Holy Qur'an, and the Messenger of Allah, peace and blessings be upon him, cursed the briber, the bribed, and the intermediary. You must forbid him from these two evil acts. If he does not desist, then threaten him with informing the employer. Then, if he does not refrain, inform the employer. He must repent and return the deducted money to the employer and the commissions to the employees, unless they forgive him for them. He must also repent to Allah for harming Muslims.