Thank you for your insight on my previous question. On daf 53b the gemara has a "stirah" regarding the "nemonus"of someone saying he made impure his friends trumah or whether he made his sacrafice pigul. abayai
answered that in the first case it is "beyado" so he is believed and in
the second case he isnt becuse it isnt "beyado". One sees from this
gemara that one isnt believed if it was beyado "lemaphraya"-retroactively. However on daf 2b tosaphos "dibur Hamschil"-ed echad he
speaks about that by schitah an ed echad is believed because it was
beyado retroactively.
By Shechitah, a person is believed because of the principle of "Ed Echad Ne'eman b'Isurim." The Shitah of Tosfos is that we need it to be "b'Yado" since what he is saying is in opposition to the Chezkas Isur. In contrast, in the cases on Daf 53-54, he needs to be believed on things that belong to someone else, and the other party stands to bear a financial loss. It is for this reason that Rabeinu Tam and other Rishonim base his Ne'emanus on the silence of the owner (see Kidushin 65b). Therefore, we need it to be "b'Yado" at the time of his testimony in order for him to be believed.
D. Zupnik