Mistake of 12.5%vat instead of 4% vat

i have purchased an item @ 12.5 % Vat and sold tha @ 12.5 vat (but the same item was of 4% Vat.)  Now material sold to the party say that the material is of 4% Vat

What to do now because  party from whom we purchased and We have filed the return.

Can we take refund if so how

pls guide me