Ruling 2340
If a person is unable to pay off the debt owed to the creditor at the time of acting as guarantor and the creditor was not aware of this but now wishes to annul him acting as a guarantor, it is problematic [i.e. based on obligatory precaution, he cannot do this]. This is especially so if the guarantor acquires the ability to pay off the debt before the creditor becomes aware [that he is unable to pay off the debt].
-Ayatullah Sistani, Practical Laws of Islam, Suretyship (Ḍamān)