Is it permissible to leave Makkah and return to one's home country due to a feeling of injustice and low salary, knowing that the questioner relies upon Allah and there is nothing compelling him to stay?

There is nothing in your question related to what is permissible or forbidden (halal and haram); rather, it is a search for virtue and what is better. It is not forbidden for you to leave Mecca even if living there is easy, nor is it forbidden for you to seek a fair salary or to forgo it and return to your country, as long as the contract does not bind you for a specific period. Similarly, it is not forbidden for you to accept a lower salary even if you find a better opportunity.

What is better varies according to circumstances and individuals. Staying in Mecca may be an opportunity for good deeds for some people, and it may be a cause for sins for others. Therefore, some scholars disliked residing in Mecca because sin there is graver. The matter also differs according to the nature of the work and the availability of job opportunities in your home country, and what temptations it may contain.

You should weigh the advantages and disadvantages of each option to achieve the greatest benefits and forgo the lesser ones, and to forgo the greatest harms by bearing the lesser ones. Consult people of goodness and experience, and more importantly, perform the Istikhara prayer, for it is one of the closest means to success.