Is Salary Payable for leave- period to non-residents taxable even if leave is spent outside India ?

Yes. The salary paid for services rendered in India is regarded as income earned in India, so as to specifically provide that any salary payable for rest period or leave period which is both proceeded or succeeded by service in India forms part of the service contract of employment will also be regarded as income earned in India and so is to be taxed.

What is Tax Free Salary and How is it taxed?
When the salary is paid 'tax-free' the employee has to return in his total income the gross salary, i.e. aggregate of the net-salary received plus the amount of tax paid on his behalf by the employer. It does not make any difference whether the tax is borne by the employer voluntarily or under a contractual obligation.

Is advanceSalary to be taxed in the year of receipt ?
The I.T. Act contemplates tax on salary which is due, whether paid or not, tax is attracted at the latest possible point of time which is the date when the salary accrues or becomes due. However, where any salary paid in advance is assessed in the year of payment, it cannot be taxed again when it becomes due. Similarly, if arrears of salary have been assessed on the 'due' basis in the past, they are not liable to be taxed again when they are paid.