Yes, it is legitimate.

It also sounds correct. Your base fare was EUR 292. Depending on the exact fare basis for each of the segments (with fees from four countries, there must be 4 or more segments in your tickets), it would be fairly common for there to be different fares in each direction. In this case, EUR 203 for the segment(s) flown, leaving EUR 89 towards a reissued ticket. By the time you pay the change fee, it may be better to abandon the ticket and recoup those taxes which are refundable and be done with that ticket and purchase new round-trip tickets with the return on those tickets pushed far enough out that you might use it.

You might also look into other carriers to see what a one-way ticket costs.

Will not be different on other carriers, it is simply that you have run into tickets with extremely low base fares.