Invoice Raised Payment Not Received

Dear Members,

X supplied services to Y and issued an invoice and now the due date for GSTR-3B has arrived,now the problem arised that whether to pay gst though he has not received payment till due date or he can pay the same on the date of realisation.
If he can defer the tax liability till the date of realisation,what is the authinticated process.
Since X is very small business having liquidity issue and liquidity purely based on realisation so how can he pay without getting amount.

Please help me in sorting out this issue