We provided consulting services to a firm for several months, everything was normal, they were super happy with our work and payments were perfectly on schedule. 

Their sales weren't as expected mid-year, so the CEO started complaining about our rates and accused us of over billing him (all the work done by us was previously budgeted and approved by him, so no over billing possible).

This company owe us about 20k USD for work completed as requested and approved.

Any suggestions?