What is the ruling on someone who, in anger over the loss of money, said, "Allah is not just," knowing that this person is a Muslim who grew up in an environment with limited religious education and whose understanding later changed?

This statement is an act of disbelief (kufr) and apostasy. Its utterer must renew his Islam. If he repents, then there is nothing held against him. It is among the Sunnah (ways) of Allah to test the believing servant, to ascertain the truthfulness of his faith and his patience, as Allah the Almighty said: "Do the people think that they will be left to say, 'We believe' and they will not be tried?" and "And among the people is he who worships Allah on a [precarious] edge. If good befalls him, he is reassured by it; but if trial befalls him, he turns around on his face [loses faith]. He has lost this world and the Hereafter. That is the manifest loss."