What are the requirements for the stipulated reward if it is paid from the property of disbelievers?

Primary text

If the reward is taken from the property of disbelievers, it is permissible for it to be unknown, provided the lack of specificity does not prevent its delivery or lead to dispute. The Prophet, peace be upon him, stipulated one-third or one-fourth of the spoils of war for a detachment (*sarriya*), which was inherently unknown because the entire spoil was unknown. This allowance is made due to necessity, as 'ja'ala' contracts are permitted according to necessity.