Excise- depot

WE HAVE DEPOT AT VALSAD AND WE HAVE EXCISE REGISTRATION NO. FOR BOTH DEPOT AND FACTORY... WE SOLD GOODS AT THE SAME RATE FROM DEPOT AND PAID EXCISE DUTY AT THE TIME OF REMOVAL FROM FACTORY.. GOODS ARE SOLD TO SSI UNITS. WE RECIEVED DEBIT NOTE(SALES RETURN )FROM SSI UNITS IN WHICH THERE IS NO EXCISE DUTY. NOW HOW TO TAKE THE CREDIT OF SAME.