Is it permissible to take a portion of money from my brother without his knowledge, with the intention of recovering a previous debt, and is this considered lawful or unlawful?

Your brother must return your right to you if he is able, because his delay without an excuse is an injustice, as the Prophet, peace and blessings be upon him, said: "The procrastination of the rich (in paying debts) is an injustice." It is not permissible for you to take your right from him without his knowledge, because that is not allowed unless he is denying the right and you have no proof, and your brother here acknowledges the right. Also, the matter should be dealt with wisely to preserve the bonds of brotherhood, even if it leads to waiving a part of the right.