[Richmond VA] A balance sent to collections

Last October I moved out of an apartment. I gave two months, written notice, moved everything out, cleaned, and turned in the key as per my lease. Around February I got an email claiming that I owed the property company ~$650 for occupying the dwelling 23 days after I had moved out. I contacted the office, they said I was fine and it was a mistake. I called the collections department (same company) and they said the account was correctly in arrears and I needed to settle the debt. I told them to contact the office and settle this themselves and to leave me out of it. Now almost a year later I have a notice in my credit report that I have a delinquent account in collections. I cannot find my copy of the original lease either. The apartment was a low income housing situation and it seems like a bullying tactic to take advantage of people in poor financial situations, thinking it would be easier to pay $600 to make them go away rather than get a lawyer involved. It's starting to negatively affect my credit score. How do I rectify this? Do I need a lawyer?