If You are purchase a Goods Rs. 10000 + Tax @ 18% Rs. 1800 = 11800.
In case You get Credit Note from Your Supplier Rs. 1000 + 180 = 1180 then You will enter as Debit Note in Your Tally. And the ITC value less from Your total ITC. But, It's not showing in Your GSTR-1 Returns... Except Document wise details.
In case You get any rate difference related Debit Note / Revised Invoice from Your supplier then You will enter it as Purchase or Journal Entry. Then Your ITC value increased. But, It's not showing in Your GSTR-1 return... Except Document wise details.
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