I left a very large organization back in Jan because an email from my direct supervisor advised to not share information about a formal audit with my customer. The audit showed that my company owed them money but we were going to advise there were no find

I worked for a very large corporation out of Nashville TN. My customer was another billion dollar organization that brought in 100 million in sales yearly to my company. There was an audit that was generated by my organization to verify the validity of rebates being submitted. The audit proved that not only were the submittals correct but that my company owed them more than $20K in back pay.
An Email from my direct boss went out stating that we should not pay back that money and that if asked we should advise them of significant findings. This was a blatant lie and completely unethical. After careful consideration I left the company for several reasons, one being constructive discharge. I could not continue to lie to my customer about the audit and its findings and there was no action taken by my employer to rectify that situation. The issue now is, my company is trying to collect for moving expenses and I am in shock that they would even consider this based on the reasoning and evidence of my departure. What are my options to fight this and avoid paying over $30,000.