VAT chargeability

Dear Friends,

here is the case in case of Tamilnadu VAT:

the client has sold a new item for Rs.20,000/-

IN EXCHANGE of an old item sold earlier valued at Rs.5,000/-

and cash payment of blance Rs.15,000/-.

Now my question is they have to collect VAT on Rs.15,000/- or Rs. 20,000/-?

please help ..thanks and regards.

Rupesh