94:20 If the Jew rented the non-Jew's place without specifying the length of time, it's valid as long as the non-Jew doesn't change his mind and he continues to reside there. If the non-Jew moves out and another non-Jew moves in, the Jew needs to rent again from the new resident. If he rented it for a specific period of time and during this period the non-Jew rented it to another non-Jew, the original rental still works. However, if the non-Jew died or sold the home to another non-Jew during the rental period, the Jew must rent again from the deceased's heir or from the new owner. If he rented from the non-Jew's servant and did not specify a time frame, it's only valid while that servant is there. If he rented it for a specific period, the rental is valid that whole time, even if the servant leaves (or dies - MB 382:45). (If the space is rented for a particular amount of time, it must be renewed, even if the non-Jew says to just let it ride - Mishnah Brurah 382:23, 26. If the non-Jew retracts, his retraction is not valid unless he refunds the payment - Rema 382:6.)
94:21 Every time the rental ceases and needs to be renewed, the eiruv must also be made again. This is because an eiruv does not renew itself.