How account only cst purchases

Hi,

In Tally i have entered the Interstate Purchases @ 2% against C Form and don't have the sales of the same to other states.

Suppose in purchase voucher i entered like this.

Cr. Supplier Account                         7,320

Dr. CST Purchases                 7,117

Dr. CST   @  2%                       142

Dr. Freight Charges                  61

Here i have take the CST @ 2% under Duties & Taxes. In the Final accounts it is showing in the balance sheet. How to close that amount of R.s 142. from the account which is CST Purchase and there is not CST Sales.