Related for Kiddushin 124:14

How [else] is this done? We do not take terumah from detached [produce] on behalf of [produce that is still] attached [to the ground] (Ter. 1:5). [If] he declared, "This detached produce shall be [considered] terumah and tithes on behalf of this attached [produce]," or "This attached produce shall be [considered] terumah and tithes on behalf of this detached produce," [it is as though] he has not said anything. But [if he declared], "This detached produce shall be [considered] terumah and tithes on behalf of this attached produce once it becomes detached," and behold, he ate from it by accident, he may take terumah and tithes from another place, until the time that it detaches [and he would not incur liability for consuming terumah or tithes]. Once it detaches, his words become permanent [and he would be liable]

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.