Responsa על כתובות 173:11

Q. B engaged A as tutor to his son. B said to A that since he, B, was unlearned and did not know whether or not A had sufficient knowledge to tutor his son, A must go to his (B's) relative C to be examined before entering upon his duties. A came to B's house and began to tutor B's son. Subsequently B discovered that A had never been examined by C. He, therefore, summoned A to court. A now declares himself ready to be examined, but B argues that A's present knowledge is no proof of his previous fitness for his position. Moreover, one witness testifies that at the time A undertook to teach B's son, he was not qualified to do so because of insufficient knowledge.
A. If A has now sufficient knowledge to teach B's son, the burden of proof lies on B that A did not have such knowledge at the time he was engaged. However, since B has one witness to support his claim, A must take an oath to the effect that he had sufficient knowledge at the time of the agreement. If A takes such oath, he will be entitled to collect his full wages from B.
SOURCES: Cr. 3; Pr. 488; Mord. B. B. 621. Cf. Agudah B. M. 172.