Bava Batra 44 - The Seller of a Field Cannot Testify for the Buyer
If one borrows money, his real property becomes a guarantee for his repayment of the loan, but not his movables. Even if he promises to repay with "the shirt on his back," still, if he does not have that shirt, he can't repay with it. However, even if he sold the field in the interim, physically the field is still there, and the lender can repossess it.
Because of that, the seller of a field cannot testify on behalf of the buyer. If someone can prove that the seller never owned the field and he is in fact a thief, then that field is not a guarantee for the seller's loans, and the seller becomes a "bad person who borrows and does not repay" - a situation that people try to avoid.