Is it obligatory for someone who took a sum of money from their company without informing its owner, and resigned without receiving their dues because they were not given to them, to repay the amount they took?

If a worker has confirmed entitlements with the employer that are equal to or less than what he took, then there is no blame on him, and he is not obligated to return what he took, because it is in exchange for his right. However, if he has no entitlements, then he must return what he took from the employer's money without permission, because the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it takes until it returns it."