What is the ruling on money saved from a salary that a brother used to pay monthly, knowing that the original salary was not received regularly? And is what he did considered unlawful? And was his termination from work due to this money?

The money your brother gave you was based on his assumption that your salary had been cut off. Had he known you were still receiving a salary, he would not have given it to you. Therefore, you must tell your brother the truth. If he is content with what you have taken, then there is no blame upon you. However, if he is not content, you must return it to him when you are able, for the Prophet, peace and blessings be upon him, said: "The property of a Muslim is not lawful except with his willing consent." As for your dismissal from work, it is not necessarily for this reason. You should renew your repentance and increase your seeking of forgiveness, so that Allah may expand your lawful provision.