Is not disclosing one's true salary to social security—for fear of it affecting the deceased father's pension—permissible, or does it involve a sharia violation, and what can be done to avoid the violation?

It is not permissible to resort to stratagems to obtain a pension without fulfilling the conditions for entitlement, nor to conceal one's true salary from social insurance to avoid deductions from the father's pension. This is because such actions constitute consuming wealth unlawfully and violating conditions. Allah Almighty has prohibited consuming wealth unlawfully, commanded fulfilling contracts, and forbidden betraying trusts. It has also been reported from the Prophet, peace and blessings be upon him, that "Muslims are bound by their conditions."