Please do not make haste to pay the differential tax to the authorities. First check whether the "C" Form is received from your customer against the invoices raised by you. If you have not received the "C" Form till date, it is advised to get in touch with the customer and see whether the customer will give you the "C" Form.
You should pay only if the customer refuses to issue a "C" Form. In that case you are advised to raise a Debit Note for the differential tax and interest thereon, on the customer against non-receipt of "C" Form.
As you can see that if you pay the differential tax from your pocket, it becomes an additional expense.
Raising a Debit Note as mentioned above on the customer will ensure that you do not pay anything from your pocket. The customer should pay the amount or the amount can be adjusted from any Open Credits in the customers account..
If you have received the "C" Form then you can aproach the authorities and submit the "C" Form. Having done so there is no liability on you for paying the differential tax.
If you are not going to receive the C Forms , the differential tax should be paid as assessment dues and the interest should be paid under the head of interest.
Hope the matter is clear now..