It's probably just a matter of saving the product information.
When you create a product with a price in euros, hikashop stores the price and the currency corresponding.
If you remove the euro currency, the price is still in euro in the database but on the listing of prices of your product, the dropdown of currencies only has the euro as possible currency so it can't display the euro currency even though that's the currency of the price. As long as you don't save the product information, the price will stay as being in euros.