Super Power/Mutation: Allen mutates his mutation away. e.g. his Innocence seemingly disappears, replaced by a normal arm.
Reasoning: Robin's arrival really confused Allen and made him think about himself -- his past -- his possibilities. If he hadn't had Innocence, and his parents hadn't abandoned him, and he'd just been NORMAL, would that have been his life? His sister? His death? So I want to say that his subconscious preoccupation with "what if I'd been normal?" has resulted in this.
Any last questions? I'm actually kind of uncomfortable with saying that this could happen -- the Innocence is technically a living thing on its own, so I don't know if I could just go, "And it vanishes for a day!" Plus it's literally in his blood, etc... so feel free to tell me this may not work.
In addition, I was thinking the curse would be very angry at the attempt to "cancel" it and it would essentially keep activating and getting suppressed, activating and getting suppressed, generally going berserk. This will have a total of zero effect since it would be distracting and disorienting and Allen would eventually just put an eyepatch over the thing.