What is the ruling on a person selling an organ from his body to pay off his debts and avoid imprisonment, in a situation where he has no source of income?

Selling human organs is forbidden. If the debtor is truly in hardship, then imprisoning him is an injustice and a violation of Allah's command to grant the one in hardship a respite. If the debtor finds no one to treat him fairly and is genuinely compelled to sell an organ (which is likely to lead one to destruction or unbearable hardship), then he must compare the two harms and choose the lesser of them, because Islamic law is built upon achieving benefits and warding off harms, and committing the lesser of two harms if both cannot be avoided. Imprisonment, if it is not lifelong or long-term, is less severe than losing the donated organ.