The first thing I'd figure out is whether or not your state's SOL has or has not expired. Look it up based on "written contracts". Most of the SOL periods are 6 years for written, but certainly can go as low as 4 years and as high as 15 years depending on where you live. If SOL already expired, then eliminate that worry from your mind.
I'd also check on the balance. It seems high. Certainly plausible in a given time frame of over a year, but you may want to do some math on that. Check your contract for the rate.
I'd do some creative rebudgeting to have the money saved to pay it off. Review your expenses. Increase income. Look to see if you'll get a tax refund. If inside SOL, they can sue and contacting them will increase the risk. However, once on a payment plan they cannot sue. Making payments won't improve your score. In fact, it'll lower your score in the long-term because each month you make a payment, they can update and mark a late on your CR (assuming Citi isn't reporting it as sold). Once you have 100% saved, then offer a PFD (you can offer less of course).
