Is the bonus offered by the employer for helping to bring in new workers, which is divided into two parts (after the new worker completes four months, then a full year), permissible or forbidden?

This reward falls under the category of Ja'alah (a contract of reward for a specific task), and it becomes due upon the fulfillment of the stipulated benefit. It is permissible to stipulate the reward based on the worker's duration of service, as conditions in contracts are valid and must be fulfilled, in accordance with Allah's saying: (O you who have believed, fulfill [your] contracts). It is a condition in Ja'alah that the reward be known in type and amount, unlike the work and the worker, whose ambiguity is excused out of necessity. Ja'alah differs from Ijarah (leasing/hiring) in that its benefit is conjectural and not binding. In Ja'alah, it is permissible for the work to be unknown if the compensation is known. Accordingly, it is permissible to take this reward upon the fulfillment of the agreed-upon condition.