If the creditor will sign a release saying that a $900 debt you owe will be satisfied for $350, make sure you get a valid signature as proof you have paid in full. Having a lawyer on your side to insure you have the satisfaction and / or release agreement document right might be something you consider.
Good luck to you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in the subject practice discipline and with whom you have an atttorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.