This plot makes absolutely no sense. These girls did nothing wrong, they are not responsible for breaking the potions in any way whatsoever, they have no reason to apologize and no obligation to fulfill this debt. Grannie is understandably upset at how the guild treated her protege, but there is absolutely no justification for her demanding that girl be her protege's slave as compensation, and there is no reason for these two girls to feel guilty since they didn't do anything. This plot is extra stupid because the protege doesn't even want slaves, he just wants to be compensated and is even willing to forego compensation to prevent this girl from becoming a slave since he doesn't think she deserves it, either.
And no, I'm not mad because slavery, I genuinely don't give a fuck about the slavery, it's that a cruel and unusual punishment is being rendered upon an innocent bystander because Grannie got her knickers in a twist and is one of the nost greedy individuals I've seen in a manga thus far and everybody just seems to be going along with it except MC and potentially beastgirl, who is annoying af because she keeps violently treating MC like the villian when he has expressed several times he is also uncomfortable with the situation and doesn't support the slavery punishment.
This is probably gonna hit the "ignore" list really fast. I don't think this plot can be redeemed, it's too retarded at this point. Unfortunate, given we're not even finished with Vol. 1 yet.