Gst excess collected from customer eligible for pmay

Dear Sir/Madam,

We charged GST @ 18% on 2/3 rd value (12% on Full value-incl land) to customer on account of sale of flat, later we came to know that that customer is eligible for PMAY CLSS scheme (which attracts 12% GST (without land Cost) or 8%(Incuding Land). 

We collected excess tax from customer and paid to the Govt. accordingly.

Now,, whether we can apply for Refund or We should raise credit note for that Excess collected & consider the same in GSTR 3B and GSTR 1.

Thanks in Advance.