From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger responds to a relative who asked why the Gemara in Bechoros challenges Rav Papa given that the Rashba's kal vachomer exempting a Yisrael married to a Kohenes from matanos might equally exempt him from pidyon haben. Rabbi Akiva Eiger argues that the Rashba's exemption operates specifically through the mekiri kehunah mechanism—the wife's implicit consent making it as if the matanos passed through her hands—which applies only to tangible items, not to monetary obligations like pidyon haben. He further argues from the Mishnah's precise language that the Kohenes case represents a fundamental exemption rather than a fulfilled obligation, confirming that the Gemara's challenge to Rav Papa stands.
Source: Shu"t Rabbi Akiva Eiger §203