Qitzur Shulchan Arukh – 65:22
It is specifically with regard to the renting of land that one may increase payment in this manner. [That is, as discussed in the previous halakhah, an agreement where “If you pay me the rent immediately, it will cost you 10 gold coins a year, and if you pay me each month, you must give me for each month 1 gold coin.”] Because land is acquired for him immediately.
But with regard to the payment of a worker, this is prohibited. That is, if he hires a person to do work for him after time, and pays him early today before he starts working, and for this the [worker] will do the work for him for less pay than it is worth — that is prohibited. Since the worker isn’t obligated from now, [the early pay] is like a loan.
But if the worker starts the work immediately, since he won’t complete the job until after a great number of days, it is permissible to pay him early so that he will work cheaply. Since he starts work immediately, it is hiring, not a loan.