The next occurrence of the word ummy we will take a look at is the following regarding the rules some followers of the previous scripture had when dealing with
[3:75] Some followers of the scripture can be trusted with a whole lot, and they will give it back to you. Others among them cannot be trusted with a single dinar; they will not repay you unless you keep after them. That is because they say, “We do not have to be honest when dealing with the ummyeen!” Thus, they attribute lies to GOD, knowingly.
وَمِنْ أَهْلِ الْكِتَابِ مَنْ إِنْ تَأْمَنْهُ بِقِنْطَارٍ يُؤَدِّهِ إِلَيْكَ وَمِنْهُمْ مَنْ إِنْ تَأْمَنْهُ بِدِينَارٍ لَا يُؤَدِّهِ إِلَيْكَ إِلَّا مَا دُمْتَ عَلَيْهِ قَائِمًا ذَٰلِكَ بِأَنَّهُمْ قَالُوا لَيْسَ عَلَيْنَا فِي الْأُمِّيِّينَ سَبِيلٌ وَيَقُولُونَ عَلَى اللَّهِ الْكَذِبَ وَهُمْ يَعْلَمُونَ
Since this is based on the rules of the followers of the previous scripture, we can investigate if there is any precedence of them claiming they do not need to deal fairly when dealing with the illiterates or the Gentiles. As expected, if we look at Jewish jurisprudence, we see no rules stating that the Jews were allowed to deal unfairly with illiterate people. Yet, we do see guidelines that Jews have different laws for how they are allowed to deal with Gentiles compared to people of the book. So, for instance, while they were not allowed to charge usury to other people of the book, they were allowed to charge usury to the Gentiles. Again, this shows that the word ummy, used in the Quran, should be understood as gentile and not illiterate.
Taken from: https://qurantalk.gitbook.io/muhammad-was-not-illiterate/ummy-illiterate/3-75-dealing-with-gentiles