From: Rabbi Moshe Sofer
To: Rabbi Betzalel
The Chasam Sofer resolves a difficulty raised by the Pri Megadim regarding the role of chezkas patur in the case of uncertain matanos kehunah. He distinguishes between a factual doubt about a specific animal — where tzadek mishelcha compels giving the gifts even in doubt — and a doubt about the person's very identity or obligatory status, where tzadek mishelcha cannot apply and the simpler principle of ha'motzi me'chaveiro alav ha'ra'ayah suffices without any chezkas patur.
Source: Shu"t Chasam Sofer YD §309