Mackieman posted:

So she's in TXL and you're in SFO, and she's coming to visit you and then you want to leave together so you can visit TXL and stay there on some sort of permanent basis? I just want to make sure I understand what you're trying to accomplish.

Assuming my understanding is correct, there is nothing prohibiting you from buying a one-way ticket on the same flight that she returns to Germany on. Do be aware that in most cases, one-way international tickets are charged at full fare, so they are significantly more expensive than the same flight as part of a round trip. Depending on the dates and the airline, you'd have to search and compare the fares and see what makes the most sense as there are cases where the round trip price does not equal the price of two one-ways.

Also consider than unless you're an EU citizen, a one-way ticket into the EU without proof of egress may cause them to not let you into the country.