What is the legal weight of the Hadith stating 'The oath is upon the defendant' in cases contradicting the oath of the claimants in murder claims?

Primary text

The established rule is that the oaths of *Qasamah* (retaliation oaths) are fifty repeated oaths, as established by sound Ahadith and agreed upon by the scholars without known dissent. Evidence supporting the claimants swearing first includes the sound, agreed-upon Hadith of Sahl, which Malik practiced upon. Arguments against contradicting narrations include that a negation does not override an affirmation; Sahl was a Companion who witnessed the event, unlike others relying on opinion; and the primary narration is in the two sound collections (*Sahihayn*). Furthermore, the tradition 'The oath is upon the defendant' was not intended for this specific scenario (*Qasamah*) because it implies people are not given what they claim without proof, whereas here they are given blood-money based on their claim. Another narration, reported by Ibn 'Abd al-Barr via 'Amr bin Shu'ayb, specifies: 'The evidence is upon the claimant, and the oath is upon the denier, except in *Qasamah*.' This specific addition is accepted because the addition from a trustworthy source is valid, and because these are repeated oaths, necessitating starting with the claimants' oaths, similar to mutual cursing (*li'an*).