Is it permissible to return money unlawfully taken from a company to its owner, who does not follow the questioner's madhhab, or should it be given as charity to Ahl al-Sunnah?

You erred in taking this unlawful money, and you must hasten to sincere repentance to Allah. Among the conditions of repentance are: abandoning the sin, regretting it, resolving not to return to it, and restoring the right to its owner. It is not permissible to give this money as charity; rather, it must be returned to its owner, whether Muslim or non-Muslim, because it is his money taken without right, and thus its return is obligatory. Allah Almighty says: "Indeed, Allah commands you to render trusts to whom they are due" (An-Nisa: 58). And He says: "And if one of you trusts another, then let him who is trusted fulfill his trust" (Al-Baqarah: 283). And the Prophet, peace and blessings be upon him, said: "Render the trust to the one who entrusted you." You can return the money without informing him of what you did.