About GSTR1

add that sale in the month of October

Don't add it in October because if u add it in October then it will reflect in the month of October in gstr2 of ur buyer whether u enter September month date it will reflect in in the month of October , Just inform ur buyer to add ur sale invoice in his gstr2 and u have to accept that invoice , But this is possible only after opening of gstr2

Yes rightly suggested by Mr Singh. Acceptance can be done in GSTR 2A Filing.