He is trying to get out of paying back to the property bank on the grounds that he has a right in the bayt al-maal

It is not permissible to try to get out of paying back to the property bank, because that is a kind of betrayal and transgression against the public wealth. Allaah says (interpretation of the meaning): 

“Verily, Allaah commands that you should render back the trusts to those, to whom they are due; and that when you judge between men, you judge with justice. Verily, how excellent is the teaching which He (Allaah) gives you! Truly, Allaah is Ever All-Hearer, All-Seer”

[al-Nisa’ 4:58]

And the Prophet (peace and blessings of Allaah be upon him) said: “The Muslims are bound by their conditions.” Narrated by al-Tirmidhi, 1352; Abu Dawood, 3594; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

He (peace and blessings of Allaah be upon him) also said:  

“I appoint a man among you to do work that Allaah has appointed me to do, then he comes and says: ‘This is your wealth and this is a gift that I was given.’ Why doesn’t he sit in the house of his father and mother and see if he is given anything? By Allaah, no one of you takes anything unlawfully but he will meet Allaah carrying it on the Day of Resurrection. I do not want to see any one of you carrying a camel groaning or a cow mooing or a sheep bleating when he meets Allaah.” Then he raised his arm (so high that) his armpit could be seen and said, O Allaah, have I conveyed the message?” 

Narrated by al-Bukhaari, 6578; Muslim, 1832. 

The bayt al-maal (treasury) of the Muslims belongs to all the Muslims, not just to a particular group of people. Those who are in charge of it are simply entrusted with looking after it and collecting it and distributing it to those who are entitled to it. It is not permissible for anyone to transgress against it or to take anything from it that he is not entitled to. If we assume that there is someone who does steal from it and transgress against it, it is not permissible for us to join him in that grave sin. If it were permissible to plunder and steal the wealth of the state on the grounds that one is taking from the bayt al-maal, there would be a great deal of evil, corruption, aggression and wrongdoing, and all of them would be guilty of betrayal.  

So beware of betrayal with regard to the public wealth, for this is wrongdoing and aggression against all the Muslims. 

With regard to his saying that the state does not give him his due from the bayt al-maal, even if this is true it does not allow him to break the covenant that he took upon himself when he signed the contract with the bank, because breaking a covenant or contract (‘aqd, pl. ‘uqood) is a kind of betrayal, and Allaah says (interpretation of the meaning): 

“O you who believe! Fulfil (your) obligations (‘uqood)”

[al-Maa’idah 5:1] 

“Certainly Allaah likes not the treacherous”

[al-Anfaal 8:58]

And the Prophet (peace and blessings of Allaah be upon him) forbade betraying the one who betrays you, so how can you betray him when he has not betrayed you?! 

He (peace and blessings of Allaah be upon him) said: “Render back the trust to the one who entrusted it to you, and do not betray the one who betrays you.” Narrated by al-Tirmidhi, 1264; Abu Dawood, 3534. al-Albaani said it is a saheeh hasan hadeeth. Al-Silsilah al-Saheehah, 423. and Allaah knows best.