BT BQ 66a(19) :ויאוש אמור רבנן דניקני מיהו לא ידענא אי דאוריתא אי דרבנן‏ : the scholars said that renunciation transfers ownership. However, I do not know if it is a Pentateuchal ordinance or a Rabbinical one
See the entire entry for mīhū conj. but, however