Reference for Bava Batra 88:2

אלא אפילו עשאו אפותיקי נמי לא מאי טעמא כדרבא דאמר רבא עשה עבדו אפותיקי ומכרו בעל חוב גובה ממנו שורו וחמורו אפותיקי ומכרו אין בעל חוב גובה הימנו

(the reason being that they are movables, and movables cannot be mortgaged to a creditor; and even if the debtor gives a written promise to pay 'from the coat on his back', that is only binding so long as they are actually there but not if they are not there), but even if he did declare them to be security, the creditor still has no lien on them.<span class="x" onmousemove="('comment',' And therefore the seller can still testify on the purchaser's behalf. ');">²</span>

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