Can anybody clarify this for me:

Lets say there is a 12,000 pound invoice and the seller wants to sell it now for 11,000 pounds. I can buy the invoice with my PPT worth 11,000 pounds and the seller gets that. When the invoice is paid back by the seller I get my PPTs back and 1000 worth of Pokens.
When I want to cash out those Pokens do I have to sell em on an exchange? I mean they are not real pounds just tokens right? Or can I swap them for "real" money on the Populous platform?