Tosefta for Bava Batra 246:9
מוקדשין לאו דידיה נינהו
and, [surely], a firstborn does not take [a double portion] in prospective [property] as [he does] in that which [was] in the [actual] possession [of his father at the time of his death]! — [The law], here, relates to the case where [the givers<span class="x" onmousemove="('comment',' Of the priestly gifts mentioned. ');"><sup>24</sup></span> were] acquaintances of the priest,<span class="x" onmousemove="('comment',' [H], Makkire Kehunah. Lit., 'acquaintances of priesthood'. Friends of the deceased who were in the habit of giving him all their priestly gifts, which, consequently, become his as soon as the beast had been killed. [Klein S., regards the phrase as terminus technicus for the 'watches' ([H]) of priests in attendance at the Temple service for one week at a time. He connects it with [H] in Deut. XVIII, 8, which is thus understood by the Talmud, Suk. 46a. V., MGWJ. 77, 185ff.] ');"><sup>25</sup></span> and [the beast] was [ritually] killed in the lifetime of the father;<span class="x" onmousemove="('comment',' Of the heirs. ');"><sup>26</sup></span>