Is it permissible to take a sum of money from the employer without his knowledge, equivalent to what was reduced from social security entitlements due to his paying a lesser amount, in order to compensate for the period you will work longer than expected?

It is not permissible to take from the landlord's property without his knowledge. Rather, one must demand the right from him or resort to the judiciary, because taking without his knowledge is a betrayal of trust. Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]," and the Messenger of Allah, peace and blessings be upon him, said: "Render back the trusts to those who entrusted you, and do not betray him who betrays you." The issue of seizing one's right is conditional on the right being established and the inability to obtain it except in this manner, and it is a matter of disagreement. The moderate view is to differentiate between whether the cause of the right is apparent or hidden.