No reimbursement from grad school for canceled open house flight expense Expedia and the airline I booked a trans-Atlantic flight with, have told me I cannot get a reimbursement. Only a refund for airline credit for a future flight. I do not need this.

The grad school I was traveling to for the open house (which was canceled obviously) is not reimbursing me because I was given the option for a refund/flight credit.

I have extremely tight funds... this flight's price was highly non-trivial on my budget.

How can I handle this?
The starting point is probably to respond politely but firmly to the academic in charge of the admissions process, pointing out that you cannot eat a flight credit, and that the grad school's actions have led to a substantial and unexpected hole in your budget. Frame it as a 'moral' obligation rather than a 'legal' one. If you've had significant interactions with any of the professors as part of the applications process, you should cc them on the email. 

If you do not get a satisfactory response, you might then escalate matters by emailing the Head of Department/Dean/University President's office. Again, being polite and non-antagonistic but persistent is probably your best option. 

If none of this gets you anywhere, your options are probably to involve a lawyer and/or a journalist.