What is the ruling on a third party settling a claim made by a claimant on their own behalf, with acknowledgment, if the claimed item is a debt?

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If the third party acknowledges the claimant's right when the subject of the claim is a debt, the settlement is invalid. This invalidity stems from the fact that the third party is buying a debt from someone who is not the debtor, making the seller unable to deliver possession. Furthermore, it constitutes selling a debt from someone other than the one legally bound to pay it. Some scholars hold that it is valid, but this view is weak because selling an acknowledged debt from someone other than the debtor is invalid, making the sale of a debt acknowledged by a denier, yet unseizable, even more invalid.