Yes, this is a problem. But we can't abandon this procedure. We have to build some mechanism so that this issue is solved. One way is that you can make a rule that if they want to sell their flat then they can only sale it, only, to the government and no their party. This way:Taking the example of Delhi, I remember 10 - 15 years back slum dwellers in jhandewalan and some more areas were given houses by government to remove the slums. But the same people sold or rented those houses and built jhuggies elsewhere. So no matter, even if government give them free houses, these slum dwellers are used to slums and will sell the house again for material gains. A salaried person works whole life to buy an apartment for his family, but these slum dwellers know, the more poverty they show, government for votes will give them free houses once in 10-20 years and without doing anything they can again become rich. These so called slum dwellers have all the amenities including TV, refrigerator, desert coolers and even AC. Giving them house to remove slum is not a solution.
1. Government will be able to retain the rooms.
2. These rooms can be used to shift some population from cities that are overcrowded to whichever city such rooms are in possession.
3. This can also help in allowing government to room bank which can be given to other families which are to be rehabilitated from slums.
Denmark uses somewhat similar approach to allow its population access to affordable housing where people can contact the government for housing in cities.