Just wanted to do a quick post for future reference. If you have a negative credit balance on one of your credit cards (i.e. from an overpayment or a huge refund that hit), most banks let you transfer that balance to another card you hold at that bank to decrease the balance owed on that card. I’ve done it with Citi and Chase.
I was chatting with Amex tonight to transfer a huge refund I got on one card to to 3 other Amex cards I have. Here is a quick breakdown of what I found: