CREDIT NOTE ISSUED WITHOUT GST

Sir,

I have a querry about Credit note issued without gst.

I have received credit note (post sale discount) against purchase from supplier "KSB Ltd." without charging GST on it.

In my books of accounts such credit note is entered as discount income without reversal of ITC. 

I have paid to KSB Ltd. net amount after deducting discount amount.

now, whether i have to reverse ITC on credit note or not. please help me with ruling and oblige.