University making unreasonable demands with regards to external undergraduate research opportunity
You need to sell this as an exception to their policy in terms of benefiting the college and it’s students. Be as positive as possible with all of this. Talk about the great opportunity you found and how it will benefit your education, further learning and your career. Make a big list of pros for why this opportunity is great for your education and for you but don’t mention cons. Not for the opportunity or for the college research. Don’t make a direct comparison. “Aboard is better because [reasons the college research sucks].” will not go over will. Stick to the positive moral high ground. Think in terms of this is right thing to do and tell them why. 

Now that you have a better opportunity, you need to convince them to allow it. Try to find anything unique to your situation that can be used as grounds to make a case for an exception. Sounds like research opportunities are scarce at the college in your field. Push that point or anything else unique. What would happen if the college research opportunity fell through or was delayed? It’s understandable to cultivate options to not miss anything in terms of your education. An exception in your case doesn't make it something that needs to be repeated for other students. Stress that point. You are presenting a unique, one time exception. Letting you go aboard allows them to build up better research opportunities while not having improvise opportunities as the need arises.  

You mentioned being in niche-field. It’s difficult for the college to give the same level of focus much less research opportunities as a more mainstream career. This makes it harder for you to find an opportunity at the college or aboard. Letting you go can benefit other students. By letting you go aboard another student has a college research opportunity.  

Reach out to your professor first. If they support you, it will be much easier with him/her on board.