And they agree that this Sufyān ibn Husayn makes errors in what he narrates from al-Zuhrī, and that reports he alone transmits cannot be relied upon as evidence. The use of a muhallil in racing has no basis in Islamic law, and the Prophet, may Allah bless him and grant him peace, did not instruct his community to use one. It is reported that Abū ‘Ubaydah ibn al-Jarrāh and others used to race for stakes without introducing a muhallil among them.

Those jurists who advocated this thought that it would otherwise constitute gambling. Some of them then maintained that introducing a muhallil removes its resemblance to gambling. Yet matters are not as they claim. Rather, introducing a muhallil involves … (1) risk-taking. Moreover, the arrangement is unjust: if the muhallil wins, he takes the prize, but if he loses, he pays nothing, whereas another contestant is given the prize if he wins. Introducing a muhallil is thus an injustice that Islamic law would not sanction.

This matter is discussed at length elsewhere. And Allah knows best.