One day, a portal, several hundred meters across, appears without warning above the Atlantic, roughly around the Bermuda triangle region. It hovers in mid air, immobile, 200 meters above the surface of the water. Space in and around it is distorted in such a way that makes it hard to directly see the other side without going through the portal, but by sending drones and eventually aircraft, it is quickly established that on the other side is one ocean of an alien planet. It's similar to Earth and has humans living on it, whose civilization is a few centuries ahead of our own in terms of technology.
Contact is established rapidly and linguists from both sides are sent to learn to talk to each other. Within weeks, the other side constructs infrastructure around the portal that can be used to transfer even the largest Earth ships through. They are clearly much more technologically advanced - this would be a megaproject for us, but it's not a big deal for them. They are generally peaceful, more so than us - while they have enough firepower that any Earth nation would be hopelessly outmatched (imagine modern Earth in contact with medieval Europe), they would for the most part prefer to trade with, learn from, and coexist with Earth. They will avoid transferring technologies that might be used for war, though. (Of course, not everyone might have such scruples.)
The question is: how exactly might trade work?
For the sake of the story, I want contact to be an extremely disrupting event for Earth humanity. On paper, their economy dwarfs that of the Earth. A back of the envelope calculation I did says it would be about 10 to 100 thousand times larger.
Therefore, in theory, they would have no problem making investments that would seem absurdly large by our standards. For example, the equivalent of a trillion dollars is not something that their governments would find all that expensive, generally. Of course, they don't have a trillion dollars: they have their own currency, and therein lies the problem. How do you buy things on Earth with alien money? How soon might people start to accept it? How would that process work? I imagine exchanges would be set up eventually, or they might trade some of their advanced technology for dollars or yuans, but it seems... hard to sell a trillion dollar's worth of stuff, no matter how fancy, for example.