GST related query

Mr. X purchase goods on credit Rs. 360000(taxable value Rs. 300000 +Rs.60000 GST) from Mr. Y , later mr. Y issued financial credit note to Mr. X Rs. 300000 after issuing that remaing Rs. 60000 GST portion is balance in the of Mr. Y as sundry debtor, it is allowed by the Mr. Y as cash discount( Rs. 60000), is any problem while giving the cash discount by Mr. Y to Mr.X? Suppose Rs. 60000(GST portion) is returned to Mr. X is any problem?
In the books of Mr. X this cash discount received (Rs. 60000) is ITC reversal is required? Suppose Rs. 60000 is received from Mr.Y is any problem itc reversel is required (because gst paid is received back through financial credit note on supplier mistake)? 180 rule applicability in this situation?