Tosefta for Pesachim 99:12

[The Mishnah, Pes. 3:8, says that one who leaves Jerusalem and remembers that he is carrying consecrated meat may burn it where he stands if he has passed Mount Scopus, but otherwise he must return to the Temple and burn it there.] What is [Mount] Scopus? [It is the place from where] one sees [all of Jerusalem] and there is nothing that obstructs [his view]. You say, one who is on his way to slaughter the Passover offering, or to circumcise his son, or to eat at a betrothal feast in his father-in-law's house, and he remembers that he has chametz inside his house, if he has enough time to return, he returns [to his house and removes the chametz], and if not, he does not return [but rather he annuls it in his heart]. Rabbi Shimon ben Elazar says, any feast [hosted by an am ha'aretz] that is not in furtherance of a mitzvah, a chaver is not permitted to eat from it (see Dem. 2:3). If he passed [Mount] Scopus, he burns it (i.e., the consecrated meat) where he stands, and if not, he returns and burns it in front of the Temple with wood from the [Altar] pile. In other words, they only said that he need not return in order to make it easy on him. How much [consecrated meat or chametz triggers the requirement that] they return? Ben Beteira says, [if] it has at least the equivalent of two eggs'-bulk, but we did not find anyone who agreed with him (see Pes. 50a:2). [If] he wanted to burn it in front of the Temple with his own wood, or on its roof with wood from the [Altar] pile (see Zeb. 107b:14), we do not listen to him.

One who purchases from a merchant (=תגר, cf. Pes. 50a:15), and he says, this one is "fixed" (i.e., tithes have been taken) and this one is not fixed, even as to the one [that he says is] fixed, we don't believe [him]. [However,] one who typically sells Demai is believed.