Get verification that the collection agency legally owns the debt.
Citibank, I am sure, will still accept payments on the account. Just tell them you owe them the money, not a collection agency. (I do that with my medical bills. And amazingly, the original creditor always accepts payments over the phone even when they can not possibly do so because the debt is in collections.)
If the OC sold the account, legally, they can not accept payment. They no longer own the account.
If you can afford the entire account, I would try a PFD with Midland. It would be much better to have it removed.
They would more than likely report paid after CO or something to that affect. It will not help you as it will still be a negative account.
And, I would be very careful accepting the settlement without specifics in writing. Such as, you are no longer financially obligated to pay the remainder. If you do not get something like this, they can come after you for the remainder.