Freeing captives: who is responsible for it?

Sayyiduna Umar (may Allah be pleased with him) said: Whenever any Muslim captive is in the hands of the disbelievers, the Muslims' public treasury shall pay his ransom.

It is narrated from Bishr bin Ghalib that Sayyiduna Ibn al-Zubayr (may Allah be pleased with them both) asked Sayyiduna al-Husayn bin Ali (may Allah be pleased with him): If a man fights in defence of the people of dhimmah and is taken captive? He said: His ransom shall be paid out of the kharaj of those very people on whose behalf he fought.

Ibrahim said: If the idolaters take the people of the covenant (dhimmis) captive, and then the Muslims overcome them, they shall not be enslaved.