Short answer. In three situations under Article 1635: when the credit was assigned to a co-heir or co-owner of the right, to a creditor in payment of his own credit, or to the possessor of the land subject to the litigated right. In these cases, the debtor cannot extinguish the credit by reimbursing the buyer.
What the law says
From the provisions of the preceding article shall be excepted the assignments or sales made: (1) To a co-heir or co-owner of the right assigned; (2) To a creditor in payment of his credit; (3) To the possessor of a tenement or piece of land which is subject to the right in litigation assigned.
Civil Code, Article 1635 — Exceptions to Redemption of a Litigated Credit. Read the full provision →
Why these are called exceptions
Article 1635 addresses exceptions to a broader rule found in the preceding article, which generally lets a debtor extinguish a litigated credit by reimbursing the assignee for the price paid, judicial costs, and interest from the date of payment. Article 1635 carves out situations where that debtor's right does not apply, meaning the debtor cannot force the assignee to accept reimbursement and give up the credit in these particular circumstances.
The first exception: co-heirs and co-owners
The first exception covers assignments or sales made to a co-heir or co-owner of the right assigned. Where the litigated credit was sold to someone who already shares an interest in that same right, as a co-heir or co-owner, the debtor cannot invoke the reimbursement remedy against that particular buyer, since the transaction is treated differently from a sale to an unrelated third party who bought purely as an outside purchaser of the dispute.
The second exception: a creditor paid in his own credit
The second exception applies to assignments made to a creditor in payment of his credit. Where the litigated credit was transferred not for cash but specifically to satisfy a debt owed to that assignee, the debtor of the litigated credit likewise cannot force reimbursement, since the assignment served to settle an existing obligation rather than functioning as a typical purchase of a disputed claim by an outside buyer.
The third exception: the possessor of the land itself
The third exception covers assignments to the possessor of a tenement or piece of land which is subject to the right in litigation assigned. Where the credit being litigated is connected to land, and the buyer is the very person in possession of that land, the debtor cannot use the reimbursement remedy against that possessor either, since the buyer already has a direct stake in the property the litigated right concerns, rather than being a stranger to the dispute.