Is it permissible to make the intention for Zakat on the amount disbursed monthly, considering the remainder as charity, knowing that the amount is sufficient to cover the Zakat?

You are permitted to intend the zakat (definition) for the amount deducted monthly from your salary before it is paid. At the end of the year, you calculate what you have paid and determine the amount due from you. If what you have paid equals what is due, then you have fulfilled your zakat; otherwise, you complete the remaining amount. To simplify the calculation of the hawl (one lunar year), you can make one hawl for all your possessions and pay the zakat when that date recurs in the following year, as paying zakat in advance of its due time is permissible.