Let us see what it (priestly gifts) is most like. We derive something (i.e., priestly gifts) that is not land-linked and not consecrated, from something (the first of the shearing) which is not land-linked and not consecrated. And this is not to be refuted by terumath reishith, which is land-linked and consecrated.
Or, go in this direction: We derive something (priestly gifts), which obtains both with much and with little (produce), from something (terumath reishith), which obtains both with much and with little. And this is not to be refuted by the first shearing, which obtains only with many (and not with one sheep).