Responsa על כתובות 152:9

Q B discovered that the horse he bought from A is injured. He claims that the horse must have been injured while in A's possession, that the purchase was a mistake and demands, therefore, that the contract be rescinded. A, on the other hand, claims that the horse was in perfect condition when he delivered it to B.
A. Even if B had not, as yet, paid for the horse, the burden is upon him to prove that the horse was injured while in A's possession. If he cannot produce such proof, A is to swear that when he sold the horse to B he did not know that the horse was injured. If he takes such an oath he is entitled to collect his money from B.
SOURCES: Pr. 575; L. 122; Mord. Ket. 200; Tesh. Maim. to Kinyan, 7; Mordecai Hagadol, p. 165d; cf. Sinai vol. VI (1942) p. 221.

Q B discovered that the horse he bought from A is injured. He claims that the horse must have been injured while in A's possession, that the purchase was a mistake and demands, therefore, that the contract be rescinded. A, on the other hand, claims that the horse was in perfect condition when he delivered it to B.
A. Even if B had not, as yet, paid for the horse, the burden is upon him to prove that the horse was injured while in A's possession. If he cannot produce such proof, A is to swear that when he sold the horse to B he did not know that the horse was injured. If he takes such an oath he is entitled to collect his money from B.
SOURCES: Pr. 575; L. 122; Mord. Ket. 200; Tesh. Maim. to Kinyan, 7; Mordecai Hagadol, p. 165d; cf. Sinai vol. VI (1942) p. 221.