_Tos'fos Yom Tov_ to _N'gaim_ 12:5 gives the following answers, if I read him right:

1. (from the _Mizrachi_ in the name of his teachers) Since he can't be sure it's a _nega_ until the _kohen_ shows up, he shouldn't say he's sure.
1. (from the _Mizrachi_ himself) He should treat the _kohen_ with _derech eretz_, etc. (The _Tos'fos Yom Tov_ includes the "etc.", thereby referring he reader to the _Mizrachi_ for a fuller explanation. I didn't check the _Mizrachi_ or, indeed, the other people he quotes.)
1. (from the _Mizrachi_ himself again) So the _kohen_ not be swift to judge it as _tame_. [I assume this means that he'll be prejudicing the _kohen_.]
1. (from the _Gur Arye_) It's not a _nega_ until the _kohen_ says it is, and he shouldn't lie. (The _Korban Aharon_ asks that that's not true: the _kohen_ says it's _tame_, but it's a _nega tahor_ even before pronounced _tame_ by the _kohen_. The _Tos'fos Yom Tov_ answers that "_nega_" in the vernacular means a _tame_ one, so he'd be lying to call it that.)
1. (his own) Because of _al tiftach pe l'satan_.