Mon Sotah 7

Rava says: If she admits that she strayed (and committed adultery) we cancel her Kesuvah by writing a receipt to the husband. Even to the Shita that we don't right a receipt (but the owner needs to produce the original document to rip up.) Here we refer to a town that doesn't write a Kesuva (therefore there is no other choice but to write a receipt, since there is no hardcopy Kesuva.)

Tosfos points out that we cannot say even if they write a Kesuva, perhaps the woman won't produce it to lose the money, so we have no choice but to write the receipt. In that case the husband can hold back the Get until she produce the Kesuva. Therefore she'll never be able to collect her Kesuva anyhow until she gets her Get.