mistake in invoice no

The below alternatives would work for period FY18-19
Alt 1 - Request Vendor to file correct invoice in upcoming gstr1 and reduce from exempted turnover in gstr1 or his annual return.

alt 2 - Request vendor to issue credit note for invoice with 12% and give a invoice mentioning 'Gst to pay under RCM'. For such invoice you pay and take credit. The same would be exempted for him.