Q. I had a medical bill that was sold to a collection agency. It agreed to let me pay $150 a month and the balance is now $1,400. But the debt has been sold to another collection agency that's demanding payment in full. Can they do that? If I continue to pay $150 a month will this have a negative impact on my credit score?

A. Unfortunately, it's perfectly legal for a collection agency to sell your debt to another and for the new agency to renegotiate any repayment plan. In fact, debt buying has become big business and many consumers are finding themselves in similar situations.

But once any collection agency is involved, you can count on any bad debt showing up on your credit report and lowering your credit score. So trust us, Experian and its compatriots know all about this unpaid medical bill.

Even though the new collection agency will undoubtedly update the entry, so what? It's an old bill and you're already suffering for it.

That means you've got to start all over and negotiate a new repayment plan with the new collection agency. Of course its first demand is for you to pay it all now. If you can't do that, tell them so and offer to continue sending $150 a month.

You've obviously done this before, but our advice on how to get debt collectors off your back provides a step-by-step negotiating strategy. It might be helpful.