Is a partnership valid between the owner of a horse and its trainer if the trainer undertakes to provide feed, care, and ride in races, on condition that prizes be divided equally between them?

Your buying horses and paying a person to care for them and prepare them for racing in exchange for half of the potential prizes involves gharar (excessive uncertainty) and Jahalah (ignorance). What the worker spends is unknown, and what he will receive is unknown and may not materialize. Therefore, the apparent ruling is to prohibit this transaction. However, some jurists have permitted it in similar situations, such as handing over livestock to someone to care for them in exchange for a portion of their milk and offspring, or handing over silkworms to someone to tend them in exchange for a portion of the silk.