The only time you're going to get a refund on a RT like that is when there is a cancellation or major delay, and you request not to get rebooked (i.e., you're making your own arrangements).

In those cases, they'll run it through refunds for the prorated amount.

Other than that, as fastair pointed out, it's not gonna happen. The best you can hope for is a date change with change fee, or maybe a like-market change in a similar region like your AMS/BRU scenario. Since the AMS return could likely have been paired with your original outbound, they can calculate the fare difference + change fee and allow that sort of change.