What is the ruling on dealing with a deficit or surplus in the company's fund, and is it obligatory to settle the deficit from private funds, especially if it is large and cannot be repaid?

You are a trustee of the fund; it is not permissible for you to take anything from it without the owner's permission. What you have taken must be returned or you must seek forgiveness from its owners. It is not permissible to issue fictitious invoices to cover the deficit, as this is fraud and forgery, which are forbidden. You must inform the officials of any shortage or surplus. If the shortage occurred without any negligence on your part, then you are not legally liable. However, if it resulted from negligence or transgression, you are liable unless the owners of the money pardon you. Hasten to repent and restore the rights. If you are unable to perform the job, then leave it.