Hi,
If you have the "Allow partners to select their currency" setting deactivated in the HikaShop configuration, then you can't change the currency for each user. Instead, it automatically uses the currency that you've selected in the "Partner's currency" setting of the HikaShop configuration.
Note that if you've disabled the EUR currency and enabled the USD currency in the System>currencies menu, you'll only see the "USD" choice in the dropdown for the "Partner's currency" setting. However, if you don't save the configuration at least once, it is still the "EUR" choice which is recorded in the database for that setting.
So make sure that you save the HikaShop configuration after selecting the currency you want for your partners.