What is the best way to reallocate an overpayment on the VAT payment screen. one of our clients has paid the determination notice for VAT and not the actual amount due?

Hello Sarah20, 

Welcome to the Community page, 

So it cant be done im afraid. You would just have an overpayment on one period and an underpayment on the other, the VAT due amount that shows when you go to record payment will reflect the overpayment.