Am I sinful for helping a friend with his accounting work when he asked me for invoices for a certain amount due to a financial deficit he has, the cause of which he does not know?

The treasurer is entrusted with the money under his hand, and he must preserve this trust. It is not permissible for him to use any part of it for his needs or the needs of others without permission. If he takes money unlawfully, he is obliged to return it, and it is not permissible for him to forge fictitious invoices, as this is a betrayal. It is also not permissible to cooperate in sin by providing fictitious invoices to help him in that. The submitted invoices must be withdrawn to prevent their use in fraud, and if he does not comply, he must be threatened with reporting him to his employer.