Is it permissible for an accountant to issue a check for a lesser amount than what the supplier demanded instead of the actual due amount, based on the financial manager's request? Does he bear legal and religious responsibility for that? And what should he do about a similar mistake he committed 10 years ago?

Rights must be rendered to their rightful owners. If it is possible to review the accounts so that each party receives their due, then this must be done. However, if the account cannot be ascertained and a specific amount is agreed upon and the supplier accepts it, there is no harm in that, provided both parties absolve each other. This falls under the category of reconciliation concerning an unknown matter, which is permissible according to the Maliki and Hanbali schools when the exact knowledge of something is difficult to ascertain. Evidence for this is drawn from the hadith (definition) of Umm Salamah (may Allah be pleased with her) regarding reconciliation over inheritances. If it is not possible to ascertain the account for the supplier, the amount must be paid to him or his heirs. If he cannot be found, it is obligatory to give the amount in charity on his behalf, with the choice for him to either take the reward of the charity or reclaim his money if he is found.