What is the religious duty of an electrical engineer working in an engineering company who suspects an agreement between a purchasing employee and a supplier to inflate prices and share the excess amount, given that invoices come from the supplier without a price and the employee justifies this by saying it is "their custom in business"?

Invoices without a price are evidence of fraud. The employee responsible for them should be advised. If the corruption cannot be removed except by informing the supervisor, then this is permissible, and it may even be obligatory. This falls under advising the company owners to prevent harm from befalling them, based on the Prophet's (peace be upon him) saying: "Religion is advice." Informing the supervisor in this case is not forbidden backbiting (namimah); rather, it is permissible advice. Imam Al-Bukhari titled a chapter: "Chapter: One who informs his companion of what is said about him," and Ibn Hajar reported that conveying information for the purpose of advice is permissible, while blameworthy is one who conveys news with the intention of causing corruption. It is obligatory to inform about the conveyance of disliked speech if it causes harm and the person spoken about is unaware of it, and it cannot be prevented except by informing. However, it is not permissible to convey news of corruption to the supervisor unless that corruption will not cease except by conveying the news. But if advising the employee removes the corruption, then it is sufficient to do only that.