If they had the money to pay, would they have been able to pay before being sent to jail?
If they didn't, how did they get the funds to obtain their release?
The goal of a debtor's prison isn't necessarily to get the debtor to pay off the debt himself, but rather to make him so miserable that his friends and family pay it for him (or at least send money to pay for food and other necessities while the debtor is imprisoned).