You have three problems:.
1. You will have great difficulty proving that the alleged agreement ever happened with disinterested witnesses. I find that oral agreements usually "never happened" when you confront the creditor.
2. You gave no "consideration" for the agreement to become a contract. Yes, you agreed to make payments and you even made payments faithfully. But you already owed all of the bill and so you did not give any new consideration for the agreement. Contracts simply are not contracts, unless there is new consideration for the agreement.
3. You do not appear to have any actual damages. You already owed the bill and the creditor was entitled to report it to the credit reporting agencies regardless of the agreement. Worse, you have not shown that you suffered any actual financial loss as a result of the report to the credit reporting agency.
This answer is not intended to create an attorney-client relationship and may not be relied upon as legal advice. A careful examination of the facts is necessary before a legal answer may be relied on. You should consult your own attorney before taking or refraining from any legal action.
1 - Probably.
2 - If you could prove it.
3 - Probably not.
You need more than a broken promise to sue. You have to show that the promise created a valid binding contract and that they breached the contract, and that the breach damaged you. You probably fail on all 3 elements.