Is the salary a doctor receives from the hospital in the month he is absent to work for another company—with a portion of his salary deducted—lawful or unlawful?

It is not permissible to work for another company during your contract with the hospital without their permission, because that is considered a breach of contract. A private employee is not allowed to work for anyone other than their employer during the contract period. Your absence for one month out of every two months without permission affects the hospital's work and causes confusion. The hospital's deduction from your salary commensurate with the period of absence is a correct action.