Qitzur Shulchan Arukh – 65:21
One is permitted to increase the rent on real estate. How is this done ?
Someone who rents him a courtyard, and says to him before he takes ownership, “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.” This is allowed.
The reason/motivation for/something one may learn from this is because from the point of the law rent only has to be paid at the end. Therefore, if he agrees to take from him 1 gold coin each month, which is equivalent to 12 gold coins [for that same year]. This is not considered as payment for delaying the rental, because he was not obliged to pay him in advance, and what he said to him: “If you pay me now, it will cost you 10” he is letting him off 2 gold coins, because he is paying in advance of the due date, and this is allowed.
If the fee isn’t due until later, the variable is saving money for early payment, not charging money for the extra time.
That line “[t]he reason/motivation for/something one may learn from this” is a nod to various definitions of the concept of ta’am hamitzvah.