Discount on purchase under gst

1.7.2017= Tax Invoice raised under GST (no discount is deducted at the time of sale) 1.7.2017=Goods delivered to purchaser by seller 1.8.2017= Payment is made by purchaser (After deducted discount which was not in the tax invoice) My question is that, the amount of discount deducted at the time of payment is taxable or not? If taxable who will pay the tax? Or the tax will be charged as reverse tax? if it will paid as RCM, ITC will be available or not?