The EU negotiates a single trade deal for all of its member nations, who are obligated legally to adhere to any such agreements, which fall largely into the arena of import duties (or lack of).
So if the EU agrees to a favorable trade deal for US soybeans, its member nations are not obligated to buy those soybeans. However, if the price is right, they would be wise to take advantage of a good deal.
Aside from the US, the only other major producer of soybeans is Brazil, who is actually surpassing the US in soybean production.
There is a problem... Brazil follows very destructive farming practices with soybeans, as this article notes. Soybeans are particularly hard on soil, extracting a lot of nutrients when grown in volume. In the US, farmers practice crop rotation - soy one year, grasses for two years to revitalize the soil. Brazil simply runs soybeans until the soil is exhausted and almost nothing will grow in it, and then cuts down more rain forest. With Brazilian soy production soaring, the deforestation has increased to a previously unheard of level.
It is true that US soybeans have been genetically modified to increase insect resistance, by increasing production of an insect repellant substance the soy plant was already making. It is also true that the EU overlooks their anti GMO stance when it's to their benefit - such as buying US soybeans in the past.
It is possible, though only anecdotal evidence to back this up, that the anti GMO position is more to benefit local agriculture than any real dangers of that process. The US and the EU have had agricultural trade spats, on and off, for decades... one well known incident was the Chicken War that occurred between 1961-1964. Understand that it is in every nation's strategic interests to keep local agriculture viable, so that there is a local source of food in the event that international commerce is disrupted by war. That can be seen as protectionism by other nations who export food. It's a touchy subject.
So one can invite the subjective issues of GMO crops, or the very real deforestation issues of Brazilian crops.
The trade standoff with China continues, but the US and EU coming to an agreement can't help their case any. It remains to be seen if the Chinese tariffs that triggered the standoff are worth more to them than the business they'll lose.