Paid/settled old bill by phone, still have balance

Made big mistake 11/05 was talked into settling a $400 medical bill for $300 payoff. Now 03/06 credit report shows $100 balance to differant collection angency. Here are some fact hope someone can help.

11/05 Received 30 day LTR from Law office listing an old CA that had been trying to collect for a few years and original DR/bill agent. Was mine I hope. Called Law office REP said if I settled today for $300 he would send me payed off letter. I wanted it first but made mistake.

01/06-03/06 Have been calling Law office to get paid off ltr with more promises. Once the receptionist told me I had a $100 balance but REP told me it that I did not. After that day received phone calls for 1 week finally called them about that New rep said I had zero balance and it would take 2-3 weeks to get ltr from different office.

3/06 Credit report shows old CA with $100 balance past due. Have not contacted them.

Question (I want to document things from here)

1. send validation ltr to Law office based on phone lady telling me I have $100 balance. with reference to phone calls being told $0 bal.

2. Send DV ltr to old CA on credit report disputing the $100 with reference to the Law office verbal agreement?

3. Do both can not count on Law office to strighten this out. I may end up paying the $100 just to get the bal to $0 but would like advice on this.

Hope some one can help thanks.