Is it permissible for him to cheat a kaafir government and take money from them?
Question

Here in one of the western countries they give a monthly allowance to everyone who comes and seeks to settle here. They say to everyone who applies: If you find work, you must tell us, so that we can stop the monthly allowance and you will have just the salary from your work. They take 30% in taxes from salaries. Is it permissible to avoid the taxes by not telling them that I have found work, thus getting both the monthly allowance and a salary? I am here for the sake of work and study, and do not plan to settle permanently.
Praise be to Allah.Firstly: 
We have quoted on several previous occasions the evidence to prove that it is not permissible for a Muslim to settle in any kaafir country except in cases of need or necessity, subject to certain conditions. 
See questions no. 3225 and 13363. 
Secondly: 
If a Muslim enters such a country, he must adhere to the conditions that have been stipulated. Allaah says (interpretation of the meaning): 
“O you who believe! Fulfil (your) obligations”
[al-Maa'idah 5:1]
It is not permissible for a Muslim to betray them or cheat them. The Prophet (peace and blessings of Allaah be upon him) said: “Render back trusts to the one who entrusted you, and do not betray the one who betrays you.” Narrated by Abu Dawood, 3534; classed as saheeh by al-Albaani in Saheeh Abi Dawood.  
And the Prophet (peace and blessings of Allaah be upon him) described one of the attributes of the hypocrite as being: “When he enters into a covenant, he proves treacherous.” Narrated by al-Bukhaari, 34; Muslim, 58. 
The Prophet (peace and blessings of Allaah be upon him) used to send his companions to fight the mushrikeen, and one of the things he advised them was: “Do not be treacherous.” Narrated by Muslim, 1731. 
This is something which demonstrates the greatness of Islam and the perfection of its laws, for it forbids its followers to be treacherous or to cheat even its enemies. 
This state has allowed you to stay there, subject to certain conditions, so you are obliged to fulfil them. 
Based on this, it is not permissible for you to conceal your work from them so that you can avoid paying taxes and take the monthly allowance when it is not permissible for you. 
And Allaah knows best. 
See also question no.  5218 and 14367. 
And Allaah knows best.