Midrasch zu Wajikra 13:17

3) (Vayikra 13:17) ("And the Cohein shall see it, and, behold, the plague-spot has turned to white, then the Cohein shall declare the plague-spot clean; he is clean." What is the intent of this? I might think that limb tips that were revealed (by recession of the nega from them) and returned to be covered (by white) would be tahor only after the (complete) blossoming of a confirmed michyah in the beginning. Whence do I derive (that he is tahor) for return of the same at the end of the first week, at the end of the second week, after exemption; after return of blossoming of confirmed white hair in the beginning, at the end of the first week, at the end of the second week, after exemption; after return of blossoming of confirmed spreading at the end of the first week, at the end of the second week; after exemption? (Whence do we derive that all of the above are tahor?) From "And the Cohein shall see it, and, behold, the plague-spot has turned to white … He is clean" — to include all (of the above).

4) I might think that one who came all white in the beginning (to be inspected) is tahor; it is, therefore, written "he is clean," but one who comes all white in the beginning is not tahor but tamei.