Is the salary earned from working in a butcher shop, whose owner practices theft in selling meat, considered lawful or unlawful, knowing that the questioner has a family to support and no other source of income?

There is no doubt that you are going through a true trial that requires patience from you. One of the greatest aids to a person afflicted with a trial is to have a good opinion of his Lord. Remember that Allah Almighty says: (And whoever fears Allah – He will make for him a way out And will provide for him from where he does not expect). Our advice to you is to stop working with this cheating butcher, because working for him is considered cooperation in sin and aggression. Strive to invite him to abandon cheating and illicit gain. If he does not respond, then leave him and look for another job, for whoever gives up something for the sake of Allah, Allah will compensate him with something better.