Salary is a payment given to a worker for doing work.

Giving Lower Rate for Salary Prepayment - If the worker was only hired to perform work in the future, it is Ribbis and forbidden to pay him in advance on condition that he will charge a lower rate for his work, since it is as if he is giving him a loan and gaining benefit from it.[1] - If the worker was hired to start working now even if it is for a long spanning project, it is permissible to pay him in advance on condition that he will charge a lower rate for his work, since he already started working, it is considered a salary payment and not a loan.[1]

[1] Kitzur Shulchan Aruch 65:22