Is the monthly deduction from the salary in a workplace, which is used to compensate workers for medical treatments, considered a cooperative fund that is permissible to subscribe to, knowing that subscription is voluntary and that the institution pays from its own funds if the amount collected from the workers is insufficient?

The institution's deduction of an amount resembling health insurance: Insurance is of two types: Commercial insurance, which is prohibited, based on gharar (excessive uncertainty/risk) and profit-making; and cooperative (Takaful) insurance, which is permissible, based on cooperation and risk-sharing. It is not possible to definitively determine the type of insurance practiced by your institution without knowing the true nature of the premiums, how they are paid, and the institution's relationship with the money.