The Rama (Rav Moshe Isserles, Cracow, 1530-1572) writes (Yoreh De’a 305:4) that if a father committed to make his Pidyon Ha’ben payment to a certain Kohen, he may not change his mind and pay the money to a different Kohen. Our Sages spoke very sternly about those who violate verbal commitments, applying to such conduct the axiom, "She’erit Yisrael Lo Ya’asu Avla" – the Jewish Nation must not act unethically. If a person gives his word to his fellow, he must honor his word, and not violate his commitment.
Similarly, the Shach (Rav Shabtai Ha’kohen, 1621-1662) writes that with regard to Berit Mila, too, once a father invites a certain Mohel to circumcise his son, he should not later change his mind and invite a different Mohel. If the father does change his mind, the Shach writes, then he is worthy of being called a Rasha (evil person).
The Hatam Sofer (Rav Moshe Sofer of Pressburg, 1762-1839), cited in Pit’heh Teshuba, goes so far as to say that even if the father told a third party that he plans on giving his Pidyon Ha’ben money to a certain Kohen, and did not tell the Kohen himself, nevertheless, he should not then give the money to a different Kohen.
The Rama adds that if one did change his mind, and made a commitment to a second Kohen, his new commitment is binding, and the first Kohen has no legal claim against him, since no formal Kinyan (legal expression of obligation, such as a handshake) was made, and the father had only given his word. Nevertheless, this is considered a grave breach of ethics.
Therefore, when a father is planning his son’s Pidyon Ha’ben, he must think very carefully before committing to a specific Kohen, as once a commitment is verbalized, it should not be breached.
Summary: When a father is planning his son’s Pidyon Ha’ben, once he committed to giving the payment to a certain Kohen – even if this commitment was verbalized to a third party, and not to the Kohen himself – it is considered unethical and sinful to then give the money to a different Kohen.