If there was nothing in the contract, loan agreement, service agreement, etc. by which you incurred the debt about having to pay collections costs or legal fees, you should be able to pay the original amount now; if you do and they still try to sue you, you could plead "accord and satisfaction" (that is, that you satisfed the debt) as a defense and likely get the action quickly resolved  in your favor. However, if you are being sued under an agreement by which you agreed to pay costs of collection or legal fees (or, for that matter, late charges or interest), you may have to pay those things to satisfy the claim; in that case, it may be too late to pay the original amount.