If an animal was conceived after the redemption of its mother it is hulin; why similarly is not milk generated after redemption - say on the third day - permitted?
Do we say that a new fetus is "panim hadashot", while the milk is the same animal?
Or do we just say that the Tora wants to avoid the regular secular use of redeemed animals, and therefore the milk and the wool is forbidden?
Yeshayhu HaKohen Hollander
As you mentioned, the Torah says that Pesulei ha'Mukdashin retain a certain degree of Kedushah even after the Pidyon (such that they are Asur in Gizah v'Avodah). We learn this from the Derashah of "'Tochal' -- v'Lo Gizah, 'Basar' -- v'Lo Chalav."