S. writes that in 2008, she owed a lot of money–about $8,000–to her dentist. She worked out a payment plan with the office, and asked them to auto-bill her credit card every month. They frequently forgot to bill her, but she wasn’t too concerned about the situation. At least, until a debt collector called her, saying that the dentist had sold her balance to them. The dentist’s office claims that this is a mistake. Now both entities want S.’s money, and she’s not sure who she should pay.