Tosefta к Трумо́т 4:5

One who intends to separate terumah [in a proportion of] one in ten, but [instead] into his hand came one-twentieth, or one-thirtieth, or one-fortieth, or one-fiftieth, or one-sixtieth, his terumah is [valid] terumah. One who intends to separate terumah [in a proportion of] one in sixty, but [instead] into his hand came one-fiftieth, or one-fortieth, or one-thirtieth, or one-twentieth, or one-tenth, his terumah is not [valid] terumah. And if he said "It is incumbent on me to take terumah," he takes terumah and [then] calculates [the amount]. [If only] one-sixtieth came into his hand, his terumah is [valid] terumah, and there is no need to add. [But if it is only] one in sixty-one [came into his hand], he needs to add more. Rebbi says, the majority of "one" [part] is like all [of one part] (i.e., one in sixty and two-thirds would be treated like one in sixty-one). How much does he add? Even one to one (i.e., even up to half terumah, see Minchat Bikkurim). Rabban Shimon ben Gamaliel says, his customary amount. Rabbi Yehuda says, he may aggregate terumah taken from the pure on behalf of the impure, and it is not [rendered] medumma, and he is not liable for the [extra] fifth. In what case does this apply? When he intends to take terumah in [the proportion of] one-sixtieth and [only] one in sixty-one came into his hand. But if he said, "It is incumbent on me to take terumah," and then he [takes terumah and] calculates, that is certain terumah [and he need not add more].