When to pay GST and file GSTR 3B?

Why party has generated  self  Invoice for payment of  RCM  are you unregistered  under GST ?.

We are registered under GST but party itself generated invoice under RCM shceme..we provide services of loan recovery agent in this case party itself generate invoice under RCM scheme. We spoke to party that we have GSTIN we will charge GST but they refused to do so.

We informed to Banks and Financial institutes that we have GSTIN but they didn't listen to us they are still generating invoices on their own under RCM scheme.

My query is that like if we provide service in August and party generating bills in September should we show that bill in GSTR 3B of August or GSTR 3B of September? Date on bill is of September.