Q: Is the rental fee due at the beginning or end of the rental period?
A: The Gemara (B.M. 65a, 110b) teaches that the rental fee is due only at the end of the usage, even though a kinyan is made at the beginning. Since the rental fee is for usage, it is due only after usage has been completed, unlike payment for a purchase, where the transfer of ownership takes place immediately with the kinyan (C.M. 126:18; Sma 317:2; Avnei Nezer, C.M. #25).
Included in rental is hiring an employee; the employer acquires the labor (usage) of the employee. Thus, wages are due at the end of the period (see Y.D. 176:6).
Nonetheless, as with other monetary law, the common rental practice is binding if not stipulated otherwise (Rema 331:1). Thus the common practice for real estate rental is to pay at the beginning of each month, not at the end. On the other hand, workers continue to be paid at the end of the month.