There is no way around the flight credit issue. If the original ticket was ex-EU issued in Euros then the rebooked ticket must be the same.
For your rebooking options, rather than throwing away half of a roundtrip or booking a one-way, etc., why not just book 2 separate roundtrips to get 2 trips to Europe out of it?
For example:
1) DFW-MAD roundtrip; DFW-MAD outbound on your preferred date, return MAD-DFW at a date way off in the future when you might be able to use it.
2) MAD-DFW rountrip (use the credit); MAD-DFW to serve as the return portion of trip #1, and the DFW-MAD at a similar date off in the future to serve as the outbound portion of trip #1.
Make sense?