Responsa for Bava Metzia 219:13

יתומים אומרים אנו השבחנו ובעל חוב אומר אביכם השביח על מי להביא ראיה

he [the creditor] must have taken great care of it, and [now] he is actually Suppressing the document, thinking, 'I will enjoy its usufruct for an additional two years.' Rabina said to R. Assi: If so, a mortgage after the fashion of Sura, which was drawn up thus: 'On the completion of this number of years, this estate shall go out [of the mortgagee's possession] without further payment:' if he suppresses the mortgage deed and pleads, 'I have bought it' — is he then believed: would then the Rabbis have enacted a measure which may lead to loss? — He replied: There the Rabbis enacted that the mortgager should pay the land-tax and repair ditches.<span class="x" onmousemove="('comment',' Round about the field, for irrigation. Hence the true ownership is known. ');">¹³</span> But what of an estate that has no ditches and is not subject to land-tax? Then he should have made a formal protest,<span class="x" onmousemove="('comment',' I.e., a declaration that the land was not purchased by the creditor. This of course had to be done before three years. ');">¹⁴</span> he answered. But what if he did not protest? — Then he brought the loss upon himself. If the <i>aris</i> claims, 'I entered [the field] on half profits'; whilst the landlord maintains, 'I engaged him on a third profits'; who is believed? — Rab Judah said: The owner is believed; R, Nahman ruled: It all depends on local usage. Now, it was assumed that there is no dispute, the latter ruling<span class="x" onmousemove="('comment',' That it depends on local usage, and since this was said in contradistinction to Rab Judah's dictum, it must mean that the aris is believed ');">¹⁵</span> refers to a place where an <i>aris</i> receives half; the former, where he receives a third. But R. Mari, son of Samuel's daughter,<span class="x" onmousemove="('comment',' V. p. 588, n. 2. ');">¹⁶</span> said to them [the scholars]: Thus did Abaye say: Even in places where the <i>aris</i> receives a half, there is still a dispute; Rab Judah ruling that the landlord is believed, since he could have pleaded, 'He is my hired labourer' or 'my gleaner.'<span class="x" onmousemove="('comment',' I.e., 'I have only hired him for a few days, and thus could have dismissed him with a small wage'; [H], here translated 'gleaner', was a sort of client or retainer (Jast.). ');">¹⁷</span> If orphans maintain, 'We have created the improvements;' whilst the creditor contends, 'Your father created them:'<span class="x" onmousemove="('comment',' A creditor of the deceased has no claim upon the increased value of an estate effected by the heirs; but v. p. 630, n. 5. ');">¹⁸</span> upon whom lies the onus of proof?

Teshuvot Maharam