Can Debt tokens be taken to pay any coin costs?
You may only take Debt tokens if a card with the Debt icon would cause a loss of coins for you.

So you may not decide to take Debt tokens instead of paying for a building (or for anything else).

Source

The rules for the Cities expansion explain the Debt mechanic on page 4. The first paragraph starts with "A card with the [Debt icon] symbol", and no other condition is mentioned.

The FAQ (also linked from the answer linked above) contains this Q &amp; A:


  Q: Can a player chooses to gain debt tokens?
  A: No, only the black cards with this symbol [Debt icon] can cause players to incur debts.