What is the reason for prohibiting the combination of a sales contract with a جعل (ju'alah - reward/commission) and currency exchange (صرف - sarf)?

The Prophet ﷺ forbade combining a sale and a loan (salaf) because a sale is based on bargaining while a loan is based on benevolence. Their combination would deviate the loan from its original purpose. Imam Malik prohibited combining contracts with different rulings, such as currency exchange (sarf) and sale, or ju'alah (a contract for a specified reward for performing a task) and sale. Currency exchange is based on strictness and immediate mutual possession, and ju'alah is based on the uncertainty of the work and the worker's option, whereas a sale contradicts these rulings. Ibn al-Arabi holds that one of the principles of the Maliki school is the impermissibility of combining two contracts that are contradictory in nature and ruling, citing the hadith that prohibits combining a sale and a loan.