Sorry. The technology Apple bought cannot be faked out by your fingerprint on anything other than your finger. It reads fingerprints subcutaneously, is immune to surface abrasions and age fading.
The technology also looks for a pulse, which your severed finger wouldn't have. Having the print match isn't good enough alone. Assuming the thief is smart enough to know its useless without your prints (which they probably aren't,) - they're smart enough to know they need it to have a pulse. And if they're willing (and smart enough to know how) to replicate a pulse, they're probably also willing and smart enough to just get your prints from other things you've touched.
+ I imagine police would take stolen fingers a bit more seriously than stolen phones and that a jury of citizen peers would find anyone cutting off fingers to be deserving of much more severe punishments.