Payment withheld in demand for epf challans

I am an accounts executive in the IT Recruitment firm.

We invoice our clients for the consultancy services of our employees on monthly basis.

A client recently withheld 2 payments out of 3 invoices raised for October 2014. The payment withheld on excuse of EPF/ ESIC paid challans be submitted for the previous month. They cleared the payment for the one invoice without the same.

Is there any law or rule as such that a vendor needs to provide EPF / ESIC challans payment confirmation to clear the invoices for the services it rendered?

Please someone guide in this regard!!

Regards,

Bips