Bukhari

Hadith Commentary:
This difference, whether it pertains to the original pledged item (rahn) or to the quantity of the pledged thing—for example, if the mortgagee (murtahin) says, "You pledged the land along with the trees," and the mortgagor (rahin) says, "I pledged only the land"—then the mortgagee (murtahin) is making a greater claim, so he must bring witnesses.
If he does not bring witnesses, then the statement of the mortgagor (rahin) will be accepted with an oath.
The Shafi'is (Shafi'iyyah) say that in the case of a pledge (rahn), when there are no witnesses, in every situation the statement of the mortgagor (rahin) will be accepted with an oath.
(Wahidi)

Hadith Commentary:
The "rahin" is the one who gives something in pledge, and the "murtahin" is the one with whom it is pledged.
This is a general principle: the claimant (mudda‘i) must present evidence or bring witnesses to prove his claim. If the claimant does not have evidence or witnesses, then the defendant (mudda‘a alayh) will take an oath that a false claim has been made against him.
The purpose of the chapter heading is to show that the same principle which applies between claimant and defendant will also apply in the case of the pledger (rahin) and the pledgee (murtahin).
A possible scenario of their disagreement is that both differ regarding the amount of the loan, while the pledged item is still present. The pledger (rahin) says, "I pledged it for one hundred," and the pledgee (murtahin) says, "I am owed two hundred by him, and this item is pledged for two hundred." To resolve such a dispute, the aforementioned method will be used.

Hadith Commentary: Benefits and Issues: This hadith establishes that taking an oath is the responsibility of the defendant (mudda‘a ‘alayh). If he takes the oath, he will be acquitted, and if he does not take the oath, then judgment will be given in favor of the claimant (mudda‘i). The claimant will not be asked to take an oath upon the defendant’s refusal to swear. This is the position of Imam Abu Hanifah and Imam Ahmad. However, according to Imam Malik and Imam Shafi‘i, if the defendant refuses to take an oath, judgment will not be given against him. According to Imam Malik, in financial matters, the claimant will be asked to take an oath; if he does so, judgment will be given in his favor. According to Imam Shafi‘i, in every type of claim, the claimant will be asked to take an oath; without an oath, judgment will not be given in his favor. However, it should be noted that in the matter of hudud (prescribed punishments), there is no oath. Regarding other claims, there is a difference of opinion about the oath, because rights are of two types: (1) the rights of Allah (huquq Allah) (2) the rights of the servants (huquq al-‘ibad).

In the rights of the servants, those financial matters in which wealth is the objective, there is consensus that an oath is required. And in those matters which are not financial, or are not related to wealth, i.e., wealth is not the objective—such as qisas (retaliation), marriage (nikah), return (ruju‘), ila’ (oath of abstention), and others—according to Imam Malik and Imam Abu Hanifah, there is no oath in these. One narration from Imam Ahmad is also to this effect, and according to Imam Shafi‘i and the two companions (sahibayn), there is an oath in these as well. However, the later Hanafi scholars (muta’akhkhirin ahnaf) have given the fatwa according to the two companions (sahibayn) that, except for hudud, in every claim, an oath can be taken from the defendant.

This is an absolute matter that proof of his claim will be demanded from the claimant. If proof—meaning any document or witness—is found, then the item will be given to the claimant. If the claimant is unable to present evidence, then the defendant will be questioned. If the defendant denies the claim of the claimant, then an oath will be taken from him. If he swears an oath, then the claimant will not receive anything. If he does not swear an oath, then the claimant will be made to swear an oath and the item will be given to him; this is called "yamin nukul" (refusal of oath). And Allah knows best.