gst on flat cancellation

we are in construction business. A party booked flat in 2015 and pay flat and service tax amount as per construction basis. we booked sale as per construction done. now in Nov2018 party cancel the flat. we are issue a flat cancellation invoice after deducting 10% of his total payment and charge Gst 18%. Now my question is how to shown in Gstr 1 for sales return and How I adjusted tax. although party have not pay any single Gst . he only paid service tax . plz suggest...