Is it obligatory to help a colleague pay the blood money (Diyya) decreed against him, and is it a condition to verify the error before providing assistance, and is it permissible to pay from zakat funds?

If the killing was accidental, then the blood money (diyya) is on the killer's 'aqila (male relatives on the father's side), and expiation (kaffara) is incumbent upon him. The questioner's brother is not obligated to pay anything unless he is part of the killer's 'aqila, in which case he would bear his share. However, if the killing was intentional, then the blood money is on the killer himself. If blood money becomes incumbent upon him and he is in financial hardship, he can be given from the zakat, from the share designated for those in debt (al-gharimin).