sales return where sales made and goods return after change in rate of tax

hii

please tell what are implications in case of sales return where sales made and goods return after change in rate of tax.

we have sold goods on 25.10.2017 @ rate 24%. these goods are return by customer on 20.11.2017 when rate of tax is change to 18%. At what rate we should issue credit note to customer 18% or 28%. tax rates are chnged by earlier GST council meeting.