Is it permissible to secretly take money from a sponsor who wrongs his worker, withholds his rights, and does not give him his full dues?

It is not permissible to take anything from the employer's money without their permission, as that is a betrayal of trust and an unlawful consumption of wealth, even if you believe you deserve a higher salary. The determining factor is the contract concluded between you. As Sheikh Al-Islam Ibn Taymiyyah said, embezzlement is not permissible based on the assumption of entitlement to an increase; rather, one must adhere to what was stipulated in the contract. If you have a proven right, you may demand it and refer the matter to the judiciary if payment is withheld.