Related for Kiddushin 124:6

Similarly, one that was coming down the road and had in his hand a basket of untithed produce [and] declared, "Behold, this [produce] is being made terumah and tithes on behalf of the produce that I have inside my house," [but] before he arrives at the city, behold, he eats from it (i.e., the produce in the basket) by accident. [In this case, he can] make it terumah and tithes on behalf of [produce in] another place until he arrives at the city. [But] once he arrives at he city, his words become permanent. If produce was eaten or stolen or lost, until he arrives at the city he the basket [is considered] tevel (i.e., untithed produce). [But] when he arrives at the city, his words become permanent. Moreover, said Rabbi Eliezer ben Ya'akov, [with respect to] produce in a garden bed, if he made it terumah and tithes on behalf of [other] produce in a garden bed, [he is not bound by his declaration] until a third of the garden bed is uprooted.