Sub rent

The rent received by your mom shall be assessed under income from house property, and the rental received by the firm for subletting shall be assessed as income under other sources.

Firstly is there any rental agreement between your mother and you. You said it is officially rented to you but then you also say there is no monetary transaction between you two. If there is no proper owner-tenant relationship in the first transaction then you cannot call the second transaction as a sub-renting. You may be merely acting as an agent for your mother, collecting rents on her behalf. 

However if there is owner-tenant relationship in the first transaction then the second transaction can be called a sub-renting. However since you'r not the owner of the property the income doesn't fall under the head 'income form house property' but under either 'business income' or 'income from other sources'. 

The original agreement between your mother and the firm must have a clause that explicitly allows the firm to sublet the premises to third party. For subletting, 30% deduction is unavailable to the firm. 

Yes. In your income from other sources you can show 50k as your rental receipts and 1k as related expenditure and offer to tax the net 49k. You'r mother will show 1k as rental income. However remember all the beneficial deductions under house property section are not available to you (such as 30% flat deduction etc). And you'r directly offering a major portion of the rental income to tax. Re-look your tax planning.

Cancel your agreement with your mom and directly let it to the third party.

If not done than it will be taxable as income from other sources of yours and under income from house property of your mother but remember that 1000 pm which you are paying currently will not be total income of her, it will be decided as per muncipal value.

so it will amount to paying tax double time.

so carefully do your tax planning.