When a card prevend the discard from another card, the effect still accours.
Examples:
Corsair Marauder (8R57) renforce tokens even if the FP have prevented the discard of the chosen possession like using Scouring of the shire
Same for Clever Hobbits 7U54 prevented from Deceit 18R29
Hmm, now that I look at the code and the rules, I don't really know if PREVENTING the discard (a cost) should cancel the effect.
The are only two entries I've foundin rules that may be considered remotely relevant, but none of them seem to directly apply to this situation:
1. If a response modifies cost, and as a result you can't pay them all, it has no effect - but the example refers to making you not able to exert a card second time, by wounding it in response to the first exertion, not preventing the exert.
2. If something prevents an effect, which would prevent another effect, the second effect is carried out - this is also a different case, as it refers to cards with template "player X can do Y to prevent Z".
So, I believe that if FP player prevents the discard of a possession when
Corsair Marauder is played, the cost is considered payed anyway, and it will add the tokens.
If you believe otherwise, please provide a relevant quote from rules.