Delayed payment of GST to MSME vendor

A private limited is delaying in payment of GST amount to MSME vendor

Above was mentioned in PO terms - Stating "GST will paid only after receipt from NHAI"

Further the company was paying the balance amount within 45 days and they are not filing half yearly return as required under section 405

Client is saying filing of return is not applicable as there is no outstanding dues

Request you to help with appropriate provision for filing of MSME return for amount due (i.e. GST on hold, retention money etc)