So if tariffs are enacted, that could possibly happen. However, there's no set date or anything, the dealer is either misinformed or lying as far as I know. The president said on June 22, "Based on the Tariffs and Trade Barriers long placed on the U.S. & its great companies and workers by the European Union, if these Tariffs and Barriers are not soon broken down and removed, we will be placing a 20% Tariff on all of their cars coming into the U.S. Build them here!"
So far it's another Twitter threat. It could become real overnight, but again, I don't believe anyone knows a specific date.
Some more info: https://www.usatoday.com/story/money/2018/07/03/car-prices-trump-auto-tariffs/745342002/
"Prices of new cars and trucks could jump by several thousand dollars in the U.S. if President Donald Trump follows through on his threat to raise tariffs on imports. The same likely would hold true even if a particular car is made in the U.S. because analysts believe automakers would spread the cost of tariffs among many different vehicles to avoid putting at a disadvantage any of their models made in foreign markets."