Do not say one more word on the subject. Not in writing, not in public, not even to your friends over beers, until you get a lawyer. Think of it like the Miranda rule -- now that a legal dispute has arisen, anything you say can and will be used by them against you.
What you shouldn't do now is panic. They've sent you a letter demanding a large sum of money? Big deal, anyone can send a letter. It's not unusual for people to try to bully others by having their lawyer draft a letter full of scary-sounding language that, when parsed by a lawyer, actually says nothing. These people hope you will read the letter, panic, and do something stupid like trying to settle with them before talking to a lawyer.
Yes. There's nothing that says you can't settle something amicably while still getting legal counsel. They've roped in their lawyer, now you rope in yours.
When you've found a lawyer, he/she will advise you on what to say and not say from that point forward. In the meantime, the safest course is to say nothing (or at least the absolute minimum you can) from now until then. If you've already sent an email you can't take it back, obviously, so when you get a lawyer be sure to give him/her a copy of the email (and any response you receive) so they can advise you on how it affects your position.
Do not say one more word on the subject. Not in writing, not in public, not even to your friends over beers, until you get a lawyer. Think of it like the Miranda rule -- now that a legal dispute has arisen, anything you say can and will be used by them against you.
What you shouldn't do now is panic. They've sent you a letter demanding a large sum of money? Big deal, anyone can send a letter. It's not unusual for people to try to bully others by having their lawyer draft a letter full of scary-sounding language that, when parsed by a lawyer, actually says nothing. These people hope you will read the letter, panic, and do something stupid like trying to settle with them before talking to a lawyer.
Don't give them what they want. Get a lawyer.