How can one deal with a mother who refuses to buy a plot of land for her son from the proceeds of a previous land sale, knowing that the father had disposed of the money from the sale of the land that the son had bought when he was spending on him from his salary, and that the mother owns an apartment and a large sum of money from the father’s inheritance?

Yes, it is permissible for the son to demand the price of the land he sold to his deceased father if the father had borrowed its price from him and did not repay it before his death. He must demand the price, not a plot of land. If the mother refuses to pay the debt, the son can sue her. As for the apartment that the deceased left in his wife's name, if it was a valid gift (he gifted it to her during his lifetime and she took possession of it), then it is her property. Otherwise, it is part of the estate to be divided among the heirs.