Tosefta for Temurah 25:32

One who purchases [produce] from the merchant and he (the merchant) says, "This is tithed" or "This is not tithed," [it is considered] certainly untithed; it is not tithed. One removes from it two tithes to exempt it from Terumah, as there is no Terumah after Terumah (i.e., once one designates a portion of certain produce as Terumah, one cannot designate any other portion of that produce as Terumah, see Tem. 13a:2).