Are commissions paid to certain parties to facilitate a company's winning of tenders or auctions considered bribes, and is the accountant who records these transactions in the accounting books sinful?

The amounts paid by the company may constitute a bribe, which is forbidden for both parties, or for the recipient only if the company is forced to pay it to obtain its rightful due. In the second case, there is no sin upon you in recording it. However, if the company pays these amounts to obtain what it is not entitled to, then both parties are sinful, and it is not permissible for you to assist in this forbidden act. You must refrain from recording anything you know to be a forbidden bribe, while it is permissible to record that which is lawful.