Responsa for Bava Metzia 163:5
לימא בדלא שוי משכון שיעור זוזי ובדשמואל קא מיפלגי דאמר שמואל האי מאן דאוזפיה אלפא זוזי לחבריה ואנח ליה קתא דמגלא עילוייהו אבד קתא דמגלא אבדו אלפא זוזי
Shall we say [that the dispute arises] when the pledge is not worth the money lent, and that they differ in regard to Samuel's dictum? For Samuel said: If a man lends his neighbour a thousand <i>zuz</i>, and the latter deposits the handle of a saw against it, If the saw handle is lost, the thousand <i>zuz</i> is lost.<span class="x" onmousemove="('comment',' Shebu. 43b. Thus, R. Akiba agrees with it; whilst R. Eliezer maintains, since the pledge is not worth the loan, it must have been meant merely as evidence of the loan. But if the pledge is worth the loan, all agree that it is a security, and therefore, if lost, the loan too is lost. ');"><sup>5</sup></span>
Teshuvot Maharam
A. Since Ri ruled that the opinion of Rabba prevails, that the responsibility of a keeper of a lost object is that of a gratuitous bailee (B. M. 82a), the responsibility of the creditor toward the excess value of the pledge is also that of a gratuitous bailee. Although some authorities are of the opposite opinion, the very fact that a difference of opinion exists creates a doubt as to the law; and in case of doubt we do not render a verdict to collect.
SOURCES: Am II, 161; Mord. Sheb. 774; Tesh. Maim. to Mishpatim, 56; Agudah Sheb. 36; cf. Pr. 185; ibid. 699b.