Two comments. Some posters indicate that FB allows to cancel the return leg of a return award and credit the miles back. This is uncommon among FFPs. Unless this benefit is explicitely stated in the T&C (which are so hard to find), it is not a "right". Hence, such a practice could be changed at any time.
As bodoy stated, in case you have made a stopover on the outbound, it cannot be treated as a one-way because the rules prohibit stopovers on one-way. You might get an agent not awaer of the rules, but that is a big risk.