If I understand your question, one simple way to do this would be to duplicate the round corner rectangle, which you said you make the perfect size for where the border starts, and enlarge it so that it's exactly the size of the whole card, or the outer edge of the card. Then select both round corner rectangles, do Path menu > Combine, then fill it with white.
That would simply cover up the design, and make it look like there is a border. However, I'm equally confused as phiscribe, as to exactly what you're asking.
Whether you simply cover up the outer edge to create a border, or use clipping or masking, you're still simply "cutting off" the design. Usually with playing cards, the design on the back has a....well it's kind of hard to explain. Let's see if I can make some quick screenshots to explain. I'll be right back.