The Secret Of על פי

The gemara learns from the words in this weeks parsha על פי שני עדים - by the mouth of two witnesses, that testimony must be verbal and not written.

Odd. There are many psukim that use the words על פי and they clearly don't mean "mouth".

The pasuk says [Vayikra 27/12] על פי אשר תשיג יד הנודר - Whatever the one who made the vow can afford.

Nothing to do with a mouth or verbal declaration.

על פי ה' ביד משה [Bamidbar 4/27] - According to Hashem in the hand of Moshe.

על פי הגורל תחלק נחלתו [Bamidbar 26/56]- According to the lot the portion will be distributed.

על פי התורה אשר יורוך [Dvarim 17/11] - According to the Torah they teach you.

So what allowed Chazal to learn from the words על פי שני עדים that testimony must be verbal?

We hope to have a shiur on this..... Stay tuned.