Ok, just to sum up then. It all started with this post:
The order of operations isn't:
1. Take debt tokens to gain debt currency
2. Spend debt currency to buy debt-cost card
It is:
1. Buy debt-cost card
2. Take the number of debt tokens specified in the cost
It is different because the first version would allow you to gain extra debt or not spend all of it. The only time you take debt tokens is when instructed (i.e. when buying a debt-cost card).
I've been trying to say that it could still be the first one, provided you would only be allowed to take debt tokens (step 1) when you're allowed to spend/pay that much debt currency (step 2) -- which is when you buy a debt cost card or when otherwise instructed (for instance overpay). In this way it would work exactly like paying coins from your pool. But it would have the side-effect of allowing unlimited debt when you overpay.
We don't know if it's the first or the second. The reason you can't overpay with debt is either: There's just a special rule saying you can't (which means it's the first). Or: "buying" doesn't mean to
pay the cost when it comes to debt, it just means to take debt tokens equal to the cost (which means it's the second).
At this point I would hope Donald would enlighten us, but I think he might have given up on this discussion.