Mortgaging agricultural land

Q: I proposed to marry a female relative of mine while in the first year of the Faculty of Commerce motivated by the fact that my mother was a 66-year-old decrepit woman and that my only sister got married. In fact, my mother suffered much in bringing us up. My father died in 1970 when I was five, my brother Muhammad was three, and my sister Khayriyah was seven years old. We only owned a small house and a leased half an acre of land. (Part No. 14; Page No. 175) With help and mercy from Allah, my mother brought us up until my sister got married, I graduated from the Faculty of Commerce in 1989, and my brother graduated from an industrial higher secondary school. My mother became very ill, old and feeble, which prompted me to propose to this relative of mine so that she might care for my mother. I, so far, have graduated from the university and am exempt from military service as I had a spleen extirpation surgery. Moreover, my brother has no job so all we do is farm the half an acre. Irritated by the long engagement period, my fiancée's family asked me to take a stand demanding no more than a bedroom which I cannot even afford. After a period of deep thinking, my brother and I decided to mortgage the plot of land so that we can afford a bedroom. My question is: is it permissible to mortgage the plot of land? In fact, some people here claim that it is permissible because of my being compelled by necessity to do so. May I mortgage the land and marry the girl which is to care for my mother after marriage or not? Am I sinful to forsake her? I ask Allah (Glorified and Exalted be He) to guide you to what is good for the Arab (Part No. 14; Page No. 176) Ummah and for all Muslims around the world. I also supplicate Allah (Glorified and Exalted be He) to generously bestow his sustenance on us.