Bava Batra 77 - Sale of a Debt Obligation
One who sells a note of indebtedness to his fellow must write for him a document of sale, which states, "Acquire the promissory note and whatever lien is in it." If he did not do so, the buyer acquired only the paper on which it is written, for use such as a bottle-stopper. The sale would then become void because of the law against overcharge.
If one says to his fellow, "Sell me this yoke for 200 zuz ," then it is obvious that the yoke alone is not sold for this amount, and thus the oxen are included. But the Sages say, "The purchase price is no proof." It may be viewed as gratuity that the buyer gives to the seller, or the overpayment, in which case the sale would be voidable.