Question Summary:
If Zakaat was overpaid previously, can it be carried forward to the following years?

Question Detail:

Is it possible to pay Zakah to a Zakah fund on a monthly basis instead of a full amount at the end of the year when payment is due?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
It is permissible to pay zakat gradually on a monthly basis.
And Allah Knows Best. 
Mufti Ebrahim Desai