That's ultimately a matter of definition. The main point is that if it is considered that Catalonia owes something to the rest of Spain, it could give that to the rest of Spain, if possible (and if it is not possible for Catalonia to give what it is considered owing, then you can argue that it is not fair to have this impossible repayment as a precondition for Catalonia's independence).

Ultimately, the core of this argument is about what could be done in terms of repayment, not what should be done. It doesn't lead to the conclusion that former colonial powers should pay their former colonies; it's about Catalonia clearing its name, so to speak.
If it's a matter of "could", not "should", then isn't it irrelevant to the matter?

Not sure what you are thinking of here.
Putting cart before horse, taking Catalonian agency for granted in musing a transaction between partners.