[The Ruling on Ju‘ālah (Contingent Reward)]

Answer — and Allah is the One who grants success: It is permissible for the reconciler to take what the giver’s soul willingly grants, as a recompense for his beneficence and a return of good with good. Allah Most High said: “Is the reward for good [anything] but good?” [Ar-Rahman:60]
It is not the right of the reconciler between the two disputants, nor between the seller and buyer, to demand a ju‘ālah and take it without the giver’s willing consent. In the hadith: “The wealth of a Muslim is not lawful except with the willing consent of his soul.”
Yes: a wage for reconciliation between disputants is permissible if the reconciliation requires going and coming and (notable) time. If there is not, in the going, coming, and time, that for which a wage is customarily due, then it is not permissible—except if it is given as a charitable gratuity with the giver’s willing consent.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2