No. Allah never does injustice to His slaves. For injustice, He must need it or he must have some interest – God forbid – to favor some of His slaves. However, Allah does not need anything. There is no injustice in testing. Allah tests everybody based on their own states, capacity and conditions.  

There are various kinds of testing in the world. Some people are tested through their wealth and some through being enemy of wealth. Some are tested through health and some through illness. Some are tested through being debtors some through being creditors...

Allah does not hold a poor person responsible for zakah but wants him to show patience. He does not undergo the testing of zakah. As for a rich person, he faces the testing of zakah. A person who does not give zakah loses the test; he is accounted for it in the hereafter and punished.   

People are not created with the same characteristics; therefore, their testing is different. Man has thousands of feelings and emotions. The development of those feelings is different in each man. For instance, the feeling of compassion in women is superior to that of men. Therefore, we understand that it is appropriate to have different tests for different people.

The reward increases depending on the difficulty of the test. If a person gets 50 points in an easy test and 100 points in a difficult test, the result might not be similar seemingly but in reality, they may be similar. One of the figures might seem high but their value might be the same. Sometimes a misfortune suffered by a person may gain him more rewards than ten misfortunes suffered by another person. What matters is to be able to understand the true nature of the question.