Accounting entry for scrap battery received during a sale

Dear Experts !!

I need help with accounting entries in GST in general. I buy battery, for example, "A1" for Rs 5000 + 28%

Then I sell this battery "A1" for Rs 5800 + 28% .

I give a discount of Rs 1500 to Customer on returning me old battery. Effectively I get Rs 5924 in hand and I have a scrap item valued at Rs 1500 in stock, which I often used to sell for cash and ignore accounting for that.

These entries were fine until now, but in GST I want to make it truly accoutable.

I want to sell battery scrap under tax invoice only. So GST on Battery scrap is 18%.

So my problem effectively is:

I buy battery at 28% GST for Rs 6400 (all inclusive) and sell for Rs 5800+28% - 1500=5924
When I sell this scrap I need to raise a bill again for Rs 1700 + 18% GST 

So should I create a seperate ledger in my books and while recording a sales transaction in tally for such a situation? (Right now, I only treat it as a discount entry)

Can I make a discount of scrab battery before adding the GST, so the implication of GST is less?

I have many issues with this scenario. I will revise my query once I am able to convince some of you experts on my concerns. Thank you all in advance.