Is an accountant sinful for knowing that the company owner is selling non-original computer supplies as original to a bank, in agreement with the bank's purchasing manager, and giving him a percentage?

You must refrain from participating with the company owner and the purchasing manager in the forbidden deception and bribery, and advise them to abandon it. If they respond, then all praise is due to Allah. If they do not respond, or if you fear harm from advising them, then inform the officials at the bank about what is happening, even if indirectly. The basis for this is the Prophet's (peace be upon him) saying: "Religion is sincere advice."