PAYMENT TERMS : "USD" I/O EUR

If your buyer want to pay you US$ instead of EUR, he may feel safer in monetary settlement. If the invoice is in US$ and if the exchange rate of EUR to US$ is getting higher, your buyer may then pay less when he sell the EUR and converted it to US$. He then has an exchange gain in converting EUR to US$. He has an expectation that the value of EUR is getting higher than the value of US$ and thus ask you for the payment term of your invoice in US$ instead of EUR.
If you used the current exchange rate of EUR to US$ for payment term, you can receive the amount of US$ with no exchange gain on the appreciation of EUR in future.
Let say, for example, if the EUR to US$ increases by 10%, your buyer could get a reduction of 10% to the cost of the goods purchased from you. He has gained an additional profit of 10%.