Citi won't refund a net credit on my account for 2 weeks

I intentionally overpaid my bill by almost $7000 in order to give me an effective increase in my credit limit. However, Citi didn't credit that to my account for 4 days, after when i needed it. I ended up using another credit card to make my large purchase after spending several wasted hours on the phone with Citi 'customer service'. Anyway, i now have a balance of almost -$7000 (meaning they owe me money) and the only way to get it back without a fee is for them to mail me a check in 7-14 days!

They claim they need to verify that it is valid. Nonsense! They have the money already. My credit limit exceeds this amount anyway. What possible reasons could they have for taking this long? Basically, they want to earn interest off this money while refusing to refund it.

Don't use Citibank credit cards!