The accused has been arrested and he has confessed to the crime, they said.
The accused Manish had constant quarrels with his mother Radha Devi over money and his love for the girl.
Upset with the son's decision, Devi had stopped giving money to him, police said.
The victim, survived by her only son lived in Vijay Vihar colony of the city since her husband had died four years ago.
She had called a lawyer to draft a will for her property, which the husband, a tehsildar had left her, they said.