Is it permissible for a man to pay 12,0 to his friend for his mortgaged land, for which the government compensated 8,0, and is there any suspicion of usury in the increase?

The state confiscated the land and paid its value to the mortgagee. Therefore, the debtor is only obligated to pay the landowner 8,0, and is not required to pay more than what was paid from the land's price towards his debt. He is not responsible for what the state did. The borrower is not the cause of what happened, so he is not liable for any decrease in price except for what was paid on his behalf from the debt. It is not permissible for the landowner to demand more than that from him, and he may demand compensation for the deficiency from the state if there is any.