You either did or did not enter into an agreement with the lawyer to settle the debt for $2,500. If there was an agreement, it either did nor did not have a deadline for payment. If it had a deadline, you either did nor did not pay on time. The agreement also either did or did not discuss when additional sums might become due. If there was an agreement, it either was or was not reduced to writing, and either was or was not signed by you.
If you did not have an agreement, then the terms of the letter offering to settle the debt either did or did not include a deadline for acceptance through payment, and either did or did not caution you that failure to reply with your acceptance would expose you to additional liability for your debt. The absence of such terms would not necessarily prevent the lawyer from telling you that a July offer was off the table if you did not try to accept that offer until December; but if you did not accept the deal, then his client's contacting you with a bill for $2,300 as payment appears to get in his way of arguing that you are somehow obligated to pay him more.
Please clarify the facts.
I'm curious -- did the lawyer's initial contact with you include the "mini-Miranda" warning: a notice to the debtor that the communication is from a debt collector and that any information obtained from the debtor may be used to collect the debt? Because it's pretty clear that he's acting in the capacity of a debt collector, even if he runs a law practice. As a debt collector he should not be threatening you with a lawsuit unless he actually intends to file suit.Quoting Biggy
If he thought he had the right to deposit your check, knowing that his client received direct payment, I suspect that he would already have done so and would instead be asking you were to refund the $600 difference between what he claims you owe and the amount he wants as payment. If he's threatening a lawsuit that he knows he will not file or that would not be valid, he's violating the FDCPA. If it were me, I might direct him to cease all contact, reminding him that his client had received direct payment in full and demanding the return of the $2,500 check.

