Bava Batra 169: In the Absence of a Guarantee

A case study of two documents for each party in the context of, for example, the sale of a field -- where two copies increases the risk of fraudulent practices, even though it sounds like a logical plan. Namely, the same business could be done twice, at least plausibly. Also, a story of one who appoints an agent to purchase land - and then the agent does so, but without a guarantee, which was clearly not the intent. So the agent has to fundamentally make up for that -- by providing a guarantee. But why wasn't the sale itself voided for the lack of fulfillment of terms?