The Gemora (Bava Basra 8b) calls pidyon shvuyim a “mitzvah rabbah” and says that captivity is worse than starvation and death. Rambam rules that he who ignores ransoming a captive is guilty of transgressing commandments such as “you shall not harden your heart” (Devorim 15:7); “you shall not stand idly by the blood of your brother” (Vayikra 19:16); and “you shall love your neighbor as yourself” (Leviticus 19:18).
And one who delays in ransoming a captive is considered like a murderer (Yoreh Deah 252:3). Indeed, Rambam himself wrote letters exhorting his fellow Jews to redeem captives and collected money for pidyon shvuyim.
Pidyon shvuyim is an absolute mitzvah, which must be followed at all times. There is ONLY one major exception, as explained in the Gemora (Gittin 45a). And that exception is either (its a machlokes) a-its too big a financial burden on klal yisroel or b-they goyim might take more Jewish prisoners. That is the ONLY exception.
Pidyun Shvuyim is a MITZVAH RABBAH, innocent or guilty.