Responsa for Bava Metzia 196:3
ואמאי מה שטענו לא הודה לו ומה שהודה לו לא טענו אמר עולא על ידי גלגול
— Said 'Ulla: [He swears] through the superimposition [of an oath]. For he [the lender] can demand, 'You must at least swear that it died of natural causes; and since you must swear thus, swear also that the hired one died.'<span class="x" onmousemove="('comment',' [H], lit., 'rolling oath,' v. supra 3a. Thus, here, the lender can plead, 'Even on your own plea, you must still swear that it died naturally, not through your negligence.' (This answer rejects Rami b. Hama's ruling that no oath is imposed at all upon bailees, even when they plead loss, theft, death, etc., unless there is also partial rejection of the claim, as above.) Since the bailee is thus bound to swear, another oath, viz., that the hired one and not the borrowed one died, is administered. The superimposed oath is Biblical, v. Sot. 18a. ');"><sup>2</sup></span>
Teshuvot Maharam
A. Since the overlord keeps the community responsible for the deficiency caused by the loss of C's pledge, the community is collectively held responsible for the entire tax (each individual, therefore, is responsible to the community and not to the overlord). Since C gave the pledge to the representative of the community, and it was lost while in the latter's possession, C is free from obligation. Therefore, the whole community must make up the deficiency. We can not compare this case to that of the man who betrothed a woman with a pledge (Kidd. 8b), and conclude that since the pledge was given as security only and did not constitute actual payment, that C did not as yet discharge his obligation; for the community is not obliged to pay the overlord actual cash, but may pay its tax in valuables as well as in cash. Thus the custom is that when a person gives a pledge to the community treasurer (and then fails to redeem it) the pledge is not sold but is handed over intact to the overlord's messenger, when the latter comes to demand the tax-money, in payment of that person's share of the tax, whether the pledge be worth much more than, or exactly as much as, that share of the tax. Therefore, whether a person pays cash or gives a pledge, makes no difference to the community. The second case you ask about, is exactly similar to the first. As soon as a person hands over his payment to the treasurer of the community, he is free from obligation. However, such payment must actually be made to the treasurer; mere setting aside of valuables as payment for taxes, is not sufficient.
SOURCES: Am II, 139.