Is it permissible to borrow a sum of money to pay Zakat in Ramadan, on the condition that the loan will be repaid from the money due in December?

There is no Zakat due on money that one only possesses in a specific month. However, if the money is owned but only received in a specific month from a reliable, non-procrastinating party, then Zakat is obligatory on it, subject to its conditions. It is permissible to delay its payment until it is received, and Zakat must be paid for past years if a hawl (lunar year) has passed over it. It is permissible to borrow money to pay Zakat if you are capable of repayment.