What does Mrs. Cratchit say when her husband asks her to drink to the health of Scrooge?

To answer this question, take a look at Stave Three of A Christmas Carol. After eating their Christmas meal, Bob Cratchit proposes a toast to Scrooge, whom he calls the "Founder of the Feast." (Bob gives Scrooge this name because Scrooge is his employer and it is his wages that have paid for the food on the table.)
In response, Mrs. Cratchit becomes angry. She reddens, for example, and says that she wishes Scrooge were here:

"I wish I had him here. I'd give him a piece of my mind to feast upon, and I hope he'd have a good appetite for it."

You'll notice that Mrs. Cratchit uses a metaphor here to reinforce her sense of anger towards Scrooge in which she compares her anger to Christmas food. She clearly harbors a lot of resentment towards Scrooge, feeling that he does not adequately support her husband.
Mrs. Cratchit, however, soon backs down. She decides to toast Scrooge for her husband's sake:

I'll drink his health for your sake and the Day's,''said Mr. Cratchit, 'not for his. Long life to him. A merry Christmas and a happy new year! He'll be very merry and very happy, I have no doubt.

Despite her anger towards Scrooge, Mrs. Cratchit has no desire to spoil the day by creating conflict with her husband.