Rav retracted from what he had said.
1. Retraction (Rav): Whether in a place where Kesuvos
are written or not, if she shows a Get, she receives
the basic Kesuvah (100 or 200); if she shows a
Kesuvah, she receives any addition to the basic
WHY HE HAD TO RETRACT? - IT'S SETTLE THE MISHNA VERY WELL!
Answer: When there is no other solution, Rav admits that
we write a receipt.
WHY WE NEED THE RECEIPT - DIDN'T WE SAY IN THE FORMER PAGE THAT SHE SWEAR- ISN'T IT ENOUGH?
Chazal do not want to set up a system where someone is likely to lose unfairly. Having to pay and being helpless against a Shevu'ah is unfair, because if one is up against a swindler (in this case the ex-wife), he has no recourse and no way to prove his righteousness. Therefore it is necessary to require a receipt in order that the husband have the proof in his hand.