Is the current situation considered a punishment from God due to insistence on selling the land to the maternal uncle who has not paid the full amount, and what should be done to recover the rights and documents from him?

If the maternal uncle is a partner in the land, his share in it is undivided and unpartitioned, and he has the right of priority to buy your share in it from the company to which your brothers intended to sell, and this is called the right of pre-emption (shufa'a). There is no sin upon you for insisting on selling your share to your maternal uncle, and you have done well in paying your brothers' share on his behalf, and he must pay you what he owes you.

You are permitted to demand the debt from him and remind him of Allah gently. If he is in straitened circumstances, he must be given respite. If he is procrastinating, you are permitted to refer his matter to the court to compel him to pay, and there is no sin in that or breaking kinship ties. Nevertheless, kinship ties must be observed, and mistakes overlooked to avoid enmity and animosity, for kinship is more important than money.