I would have gotten the opinion from the lawyer before putting ANYTHING in the mail--any time you make a payment that is a form of admission that you owe a certain amount, and better to be sure before you do it. That said, it isn't a huge amount so I don't think you've shot yourself in the foot, but ignore any further emails from him on this issue.
In the future, don't debate it with him. It's easy for an abuser to get one tangled up in knots. Your position needs to be, "I'm following the order." PERIOD. Don't listen to his arguments and you won't be doubting yourself. Listen to the EXPERT that you HIRED.