employer and contractor diaputs

The query lacks clarity as employee relations are different from contractual obligations.  There is no clarity as to whether you are an employee (Purchase officer ?) and what is this contract between employer and employee apart from salary or incentive or some other kind of reward ?

You can proceed as per the terms of Purchase Order. Just go through the terms and conditions and act accordingly,if there are no terms and conditions then issue him a legal notice for your amount against the deliverd P.O. and claim damages too.

Yes, the posting lacks clarity.