Export bill claim

QUESTION : FOR AN EXAMPLE WE ARE EXPORT KNITTED GARMENTS EUR 50000. BUT OUR BUYER PUT A CLAIM FOR THAT PARTICULAR GOODS. BUYER SUPPOSE TO PAY ONLY EUR 40000. BUT WE ARE ALREADY RECEIVE A DRAWBACK AMOUNT FOR THIS BILL TOTAL AMOUNT EUR 50000. IN THIS CASE WE ARE NEED TO PAY REVCEIVED EXTRA AMOUNT TO THE CUSTOMS. IF IT IS YES OR NO? WHY?
AND ALSO HOW TO CALCULATE THAT AMOUNT. ANY INTEREST FOR THAT.