All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First, the work contract between your father and that farmer is permissible; for more benefit, please refer to Fataawa 86992 and 86803.
Second, the fact that the bank placed a lien on the land of your father against the money he borrowed from it is also permissible according to the most preponderant opinion of the scholars .
For instance, Al-Mardaawi from the Hanbali School, said: ‘It is permissible to have a lien on a rented property according to the most correct opinion of our School (Hanbali), and this is the view of the majority of the scholars of the School. However, other scholars said that it is not permissible.’
Third, regarding the Zakat on the income of the land, it is an obligation to pay it if the income reaches the Nisaab (i.e. the minimum amount subject to Zakat) and if a whole lunar year elapses while possessing it. The fact of being indebted does not exempt the person from the obligation of paying Zakat if your father has other wealth from which he may pay the debt.
The only case when the person who is indebted may be exempt from paying the Zakat –according to the view of some scholars - is when the indebted person does not have surplus amount of money over the money needed for his personal expenses, an accommodation of a person of the same social status like him, a means of transportation, and any other necessary items.
Allaah Knows best.