Dear Puja,
Legality of retention of half month's salary can be assessed depending how it is communicated to the employee. Suppose while issuing the appointment letter, it is mentioned that for the first month, the salary will be "X" and second month onward it will be "2X" then I do not think that it can be questioned. This is because terms and conditions of employment are specified in the appointment letter and employee accepted those. Therefore,grumbling at later stage is not worthwhile.
Going further, many employers communicate about retention of 50% salary at the time of recruitment itself and later they make this provision of deduction and disbursement after one year in the letter of appointment. Will any law prohibit doing this? As long as minimum wages are paid in the first month, everything is fine!
Thanks,
Dinesh Divekar