What letter is given by a collection agency after a debt settlement

You can check your credit report to know if your debt has been settled. Your account status will be updated as "paid as agreed" or "paid as settled". Get a written settlement agreement before sending money to the CA.

I agree with both of them, check your credit report to get the proper status of your account. If it is "paid as settled" then you don't have to worry anymore :)
You can find few samples of debt settlement letters here - https://community.debtcc.com/letters/index.html

Check your credit report to know whether or not it is updated as "paid as settled". Or you can contact the concerned settlement company to get the update.