Compliance Formalities - clients insist on seeing salary slips of our service engineers?

Some of our clients insist on seeing salary slips of our service engineers & also wages register. These are confidential matters & am not comfortable sharing this info. Is this mandatory to provide these details ? if yes, under which rule ?
14th November 2014 From India, Mumbai

The salary slip is to be given to all the employees of any organisation. It is the duty of the Principal Employer to ensure that the payment is made in time and the Minimum wages is complied. Hence as a contractor you must produce the copy of the Wages Sheet and the Wages Slip, PF Challan, ECR Uploaded copy with the employees engaged by you in their establishment, ESI Challan on monthly basis and the Statutory Returns being submitted to the appropriate authorities to the Principal employer on demand.
14th November 2014 From India, Kumbakonam

Dear Manoj
What do you mean by Service Engineer. Is he your employee engaged with Contract of employment or he is coming from the Manufacturer, only for servicing the equipment you owned with Contract for employment. If he is your employee then obviously you must comply with all the statutory obligations and on demand you must provide the details being asked by your Principal Employer.
16th November 2014 From India, Kumbakonam