A creditor of a debt that the decedent had at the time the succession opened may, even where a designation of shares of the estate has been made pursuant to the provisions of the preceding Article, exercise the creditor's right against each coheir in accordance with the share of the estate calculated pursuant to the provisions of Articles 900 and 901; provided, however, that this does not apply if the creditor has approved the assumption of the debt by one of the coheirs in accordance with the designated share of the estate.