The conversion of price between multiple currencies will be done by the multi-currency engine. Let's taken an example of the Material product we have. The sale price is $100. When we sell in Europe we will raise an invoice in EUR; this computes the price to 89.74€ based on the rate. However, some business cases do not require the price conversion, but a fixed price that is not based on the rate conversion, such as where the price of a t-shirt in the USA is $100 while the same t-shirt sold in Europe is 90€, which is not a conversion between USD to EUR.
The only way is to create a Pricelist, for EUR currency and set the fixed price on the product. You can get the price of the product depending on the Pricelist and not based on the currency conversion:
Create a sales quotation in EUR. Try to sell the Material product. Instead of rate conversion, you should get the price of 90€. Look at the sales order screen here: