Link makes a vague, negative noise - unfortunately, unfusing isn't actually something he can do. Once something's on, it's on.

Though, that doesn't mean he doesn't have options.

Taking the now fused shield, he steps up to a nearby wall. And then bashes it, violently, several times against said wall. At which point the crate promptly collapses into a collection of tiny, tiny pieces.

He steps back towards Donnie. Now it's good to go!