If builder is collecting 100 rupees rent per month that means he has purchased property from the erstwhile landlord and he himself has become landlord of the property.
Now he is the owner of the property and developer as well.
The buyer, you (your father) and the builder if he has purchased the property, all three of you have to enter into an assignment deed/tenancy agreement under which you will relinquish your right in the flat and the buyer will acquire your existing rights in the flat with the concurrence of the landlord. Usual payments may be entered in agreement and it will entail payment of stampduty and registration also.
Naturally landlord will have his share as per mutual understanding.
For income tax ask your CA.