Related על קידושין 123:17

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]