We have a contract to pay for Approved TimeSheets. This client is asking to pay for w/o approved TS. And saying prev invoices paid w/o ?

We have a consultant Antonio hired thru MobileSolutions ( Antonio's employer, Atlanta based) and Antonio worked at one of our client GlobalLog. We have an agreement which states that "MobileSolutions will be paid for all TechnoHarbor( means us) or client (means GlobalLog) approved timesheets. " We used to get approved timesheets ( approved by a project manager at GlobalLog ) from Antonio and we paid the invoices from MobileSolutions. The last invoice we never received any approvals and MobileSolutions is demanding to pay that invoice ( $3200 ) and saying that we never asked them for approval proof previously. In fact we send several emails for this last timesheet to both the MobileSolutions and Antonio. Now their inhouse legal want to go to court. We also want to see if we can hire an in house legal for our future needs as we are in IT Staffing and have on going need to look at contracts etc for a monthly retainer or similar arrangement.