Payment voucher and self invoice when to issue

I want to know whether I have to issue payment voucher and self invoice  in case of every RCM transactions or only in case  supplier is unregistered and  please correct me if am wrong   for e.g suppose I received advance of rs 50 from my buyer against order of rs 100 then i will pay gst @18% on rs 50 i.e rs 9 and i will charge GST @18%  on balance rs 50 i.e rs 9 when i prepare final invoice