Responsa for Bava Batra 14:4
אמר ליה רב אחא בריה דרבא לרבינא הלכתא מאי אמר ליה בכולהו הלכתא כרב חמא לבר מתברא דסהדי בשקרי לא מחזקינן
against the other; and take good heed of these rulings, because they are firmly established. Raba, however, said that each has these rights against the other. There was a bond [inherited] by orphans [from their father] against which a receipt was produced [by the borrower]. R. Hama said: We neither enforce payment on the strength of the bond, nor do we tear it up. 'We neither enforce payment', because a receipt is produced against it, 'nor do we tear it up', because it is possible that when the orphans grow up they will bring evidence invalidating the receipt. Said R. Aha the son of Raba to Rabina: What is the accepted ruling in such a case? — He replied: In all [the above-mentioned cases] the law follows R. Hama, save only in the matter of the receipt, the reason being that we do not presume the witnesses [who have signed the receipt] to have been guilty of a falsehood. Mar Zutra the son of R. Mari, however, said that in this also the law follows R. Hama, since if the receipt were genuine the defendant ought to have produced it in the lifetime of the father, and since he did not do so, the inference is that it was forged.
Teshuvot Maharam
A. Matters of taxation depend on local custom. It is needless to ask me the custom of our community, for you may have a different custom. Anyone who settles in a new place is bound by the custom prevailing there. But since the community has completely settled its obligations to the burghers, by consenting to pay the tax to A, before Leah settled in T, the representatives of the community must prove through witnesses who are not residents of T, that according to their custom Leah was obliged to pay the amount the community demanded, before they could deprive her of any of her property. If Leah, however, rented a house in T before the tax was levied, she is bound to pay her share of the tax, despite the fact that the community may not be able to prove any prevailing custom to that effect.
SOURCES: Pr. 995. Cf. Menahem of Merseburg, Nimmukim (8).