Tosefta for Bekhorot 59:15

One who sends fruits to his friend, and he [presumably the friend] decides to return it, he comports himself [vis-a-vis the returned fruit] according to how it was (i.e., its prior status). Abba Shaul says, "I would be concerned lest it be switched." Rabbi Shimon ben Gamaliel says, "If he has a sharecropper who knows how to tithe, but he (i.e., the landowner) does not trust him as to tithes, and he (i.e., the sharecropper) brought his fruits to him, and he said, yours (i.e., your share) are tithed, he is believed [in this instance]." Rebbi says, "He is not trustworthy [according to the principle] that one who is suspected regarding [one] matter may neither adjudicate nor testify" (Bekh. 30a:3).