Payday lender bought old payday lender and wants more money

I had received a payday loan in colorado springs CO back in Nov 2011 for $500.00 full payoff amount was 792.54. the lender was AAFA of colorado inc d/b/a First American Cash Advance. I had paid my instalment of 132.09 from Dec and Jan however in Feb I was unable to go into the ofc and pay due to my disability (unable to drive) therefore I had informed them to go ahead and deposit the last check i made for them of the amount of 510.45 which they did but not till Feb 2/21/12. I was informed by the clerk that I would be paid in full. I assumed all was taken care of until I received a call from Advance America stating they had bouth First American and I still owe a balance due to them not depositing my check till Feb. I was NEVER informed of additional interest at the time I told them to cash the check. She stated I was paid in full. But since this was over the phone I never received a paid in full receipt. I have just now started to get phone calls from the new company. Do they have any legal right to go after me especially if I am just NOW becoming aware of additional interest and wasn't aware that I owed any more money especially since I never got the loan from them originally it was with First American cash advance NOT Advance America who is now contacting me. Any information would be appreciated.