Madam, As per Rule 36(b) of CGST Rule Credit of RCM can be taken upon payment of same. hence if you filing August month return and for August month you have RCM liability of Rs 5000 then add this amount to Output Tax Liability and simultaneously take credit of same in same month. when you go for filing August month return you will be asked to pay Rs 5000 in cash first. So when you pay this amount in August itself or in September you have complied with Rule 36(b) and hence you can take credit in same month I.e while filing return for August.