At what rate invoice should be issued?

Nischal Mittel (43 Points)

23 February 2018  
A registered dealer purchases a leather at a rate of 12 percent gst on it. Now he returns the goods to a supplier afterwards. On that date the rate of gst on leather changes from 12 to 5 percent. Now my query is at what rate debit note should be issued to a supplier?

please reply ASAP