"if you do not redeem it, you shall break its neck": Since you have caused a loss to the Cohein, you, too, shall suffer a loss. And whence is it derived that benefit may not be derived from it? It is written here "breaking," and elsewhere (in respect to the heifer of the broken neck [Devarim 21:4]), "breaking." Just as there, no benefit may be derived (from the heifer), so, here, (in respect to the first-born of an ass).