Most important, get it on writing!
First, a 50% lump sum is not such a good deal for a debt this age. You can settles for less, between 25% – 35% of the original debt. Remember – they probably bought your debt for a few cents on the dollar.
The damage to your credit is already done, and paying the debt won’t raise your score. Unlike credit card debts which are reported by both the credit card company AND the collection, medicals bills are reported to your credit report only by the collection agency. This means that if you can negotiate a Pay for Delete Agreement, it will improve your credit. If you agree for 50% settlement, demand a pay for delete.
In any case, have them send you a written and signed agreement. The people who get more demands from collection agencies after making payments are the ones that do not have a written agreement. Pay only with Money Order or Cashier’s CSheck. Never give them your checking account number. Keep that agreement and your payment proof forever. See how-to-pay-off-debt.html for more tips on dealing with debt collectors.