Ok let me chime in on this one.

First of all, you specifically stated slave and Master. As Denu mentioned, slavery implies a whole other set of factors, namely, a slave does as she is told, and hard limits technically do not apply in the dynamic based on the most widely accepted definition of a slave.

Now assuming your hypothetical is real (because i obviously dont know the real situation), and on the assumption the person involved is a slave, unless there is a valid medical reason why she should not even take one sip at the toast to her marriage, I would not accept it as a valid hard limit. Taking one sip of alcohol to most people is not going to kill you or cause any kind of mental trauma - valid reasons why a Master shouldn't inflict something on a slave.

In terms of how the situation is approached, if my slave said to me "I am not doing X" then I think I would have a serious problem with that, especially in the way it is described in the hypothetical, it comes across as rude, disrespectful and if it had happened to me would have been entirely unacceptable.

How you ask for things is everything. If in the hypothetical the slave had said "Master would it be ok if I toasted with a non alcoholic drink" that would come across a lot better than "I am not toasting as I don't drink (and to hell with you if you want me to do otherwise)". See the different in context? Being a slave does not mean you don't ever get your own way, but how you ask for things and request things is everything.

Regards,

Arch

p.s. this answer would be entirely different if the person in the hypothetical was a submissive, not a slave.