All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
It appears from your question that those from whom you buy goods for the company write a price on the invoice that is higher than the sale price so that you take the additional amount; this is not permissible because you did not take a commission from the seller here; you took it from your employer and cheated by writing that on the bill [by adding it on the bill as if it was the original price of the item]; and it is confirmed that Abu Hurayrah narrated that the Prophet said, "Whoever deceives us is not one of us."
Abu Hurayrah also narrated that the Prophet said:,"Whoever cheats is not one of my followers." [Muslim]
Besides, At-Tabaraani reported another hadeeth with the following wording: "Whoever deceives us is not one of us, and deceit and cheating are in the Hellfire.”
Therefore, you have no right to do that unless you inform your employer about it so that there will be no deceit and no consuming of the company’s money unjustly; if your employer agrees, then you are not sinful, otherwise, it is not permissible for you to do that by cooperating with the seller on cheating and forgery.
Allaah knows best.