Hermitage Punishment Room
Brand takes the potion and carries it to Waterstrider. Since the Wolfen is already standing, he can only hand it to him, giving his paw a squeeze - ostensibly to make sure he has a grip on it before Brand lets go. It's the only support he can offer his friend. Trying to make eye contact would be too conspicuous with their height difference. Too dangerous, as it might lead to Brand bursting into tears of guilt.
He waits until Waterstrider has finished drinking it, then follows the guard to the kitchen. Unless there's an obvious place in the punishment room for used potion bottles, he'll carry it with him to wash the bottle while he's preparing the meal.
Of course if Elizabethele direct him to place the flask anywhere else, he'll obey.
@V: I think you mean Brand, not Justin.