With due respect to @O. Abdul Hameed, @Raj Kumar, @Sh Srinivas, and other members, I wish to add that such cases of deduction from employee salary as "Check off Facility" work well when there is a large employee strength, where the union finds it practically difficult to collect the funds from their members. As rightly pointed out, consent in writing is a must from members and has its own additional HR process for the company to maintain such documents.
Practical problems arise when there is a dispute by the worker with the union or a new union is formed, and other issues that may arise later. If management agrees to deduct such payment on behalf of the union, I am sorry to say, but it’s like asking for trouble for oneself by gaining nothing. So, always the safe legal counsel advice to management is "AVOID." Provided the company has an employee strength like that of National Insurance Company or Indian Railways, etc., who are spread across the country, such things work as it has its own dynamics and benefits to employees as well.
Regards,
Ukmitra