Updates the outstanding debt amount (debtSum) of an ACTIVE or DISPUTED payment default.

The amount cannot be updated for payment defaults with ENDED or PENDING status:
- For ENDED payment defaults, the amount cannot be changed.
- For PENDING payment defaults, the creditor must delete the existing payment default and submit a new one with the updated amount.

A 422 Unprocessable Entity error is returned if:
- debtSum is missing, not a valid decimal, or outside the allowed range (must be at least 0.01 and less than 1,000,000.00).
- The payment default status is ENDED or PENDING.

A 404 Not Found error is returned if no payment default with the given ID exists.