How to apply GST to Purchase discount

Hi,

We receive materials from supplier with cash discount.

Example

Item Value                           Rs.100000

GST 5%                              Rs.    5000

Discount .5% on Item cost   Rs.     500

We pay to supplier Rs.104500 (100000+5000-500)

Here GST is calculated before discount, we need to calculate GST After discount. How to calculate the GST after deduct the discount in Purchase invoice.

Thanks

Mohan