Theory:

     Obviously Toto was not the sort of pet we could keep for long. Even Grandfather realised that. We were not well-to-do, and could not afford the frequent loss of dishes, clothes, curtains and wallpaper. So Grandfather found the tonga-driver, and sold Toto back to him — for only three rupees.
Explanation:
 
Grandfather happily brought Toto to raise, but things did not go as planned. He finally realised that Toto, like his other pets, couldn't be kept at home. Grandfather initially liked and appreciated its activities. He was never against Toto and hated him, but he quickly realised Toto's nature.
 
The mischievous Toto.jpg
The mischievous Toto

The narrator also makes a valid argument here, stating that, while the narrator or his grandparents did not hate Toto, they couldn't afford to replace items like plates, curtains, and wallpaper regularly since they do not come from a wealthy family.
 
Finally, the narrator's grandfather decided to take Toto away from their house. He was able to track down the tonga-driver and sold him, Toto, back for just three rupees as he somehow wanted to get Toto out of their home.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Realise
Become fully aware of something
2.
Well-to-do
Wealthy or prosperous
3.
AffordHave enough money to pay for
Reference:
National Council of Educational Research and Training (2006). Moments. The Adventures of Toto – Ruskin Bond (pp. 7-10). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.