Tosefta על בבא בתרא 246:4

Associates of the priesthood and of the Levites (those with an arrangement with a specific Kohen or Levite to give him their priestly or Levitical gifts, see Tosafot to Bava Batra 123b:8) are not authorized to do this; however, [with respect to] lost items, it is permitted because he is like one who returns a lost object [to its owner].