Sales Tax on Bad Debts

[khizar]

Hi

Say I billed a client for Rs 100,000 plus the 15% GST = 115,000.
Over the next 4 months he paid Rs 105,000. Balance of 10,000 is unpaid, claiming that there were some rejections / incorrect rates etc.

I guess the 2 options are
1) Issue a credit note for Rs 10,000 (incl of GST)
2) Bad debts of Rs 10,000.

If we treat it as bad debt, will the total 10,000 be shown as expense. Or just the revenue portion, balance crediting GST?
And to justify the bad debt, are any documents / proofs required?

Is it true that credit notes cannot be issued after 90 days for any invoice?

Thanks