Most times you will notice on a debt collection letter that the debt collector will settle the debt you owe for a lower amount if you can pay the full amount of the lower amount of debt off.
That means if you have the lump sum of the money that the debt collector is willing to accept to clear your debt then you won't have to pay the full amount of debt you owed back.
But if you can only make payments to the collection agency then you'll have to pay the full amount.