This is one of those situations where, in my opinion, there are several different ways of expressing this idea.
Here's how I would translate your example:
As I'm sure you're aware, the phrase "You know what?" is an English expression that the Japanese would find funny if you translated it directly. In my experience, if you say something to the effect of "now that I think about it..." you'll be able to communicate the idea that you have just made your made a decision. Just know that in the situation above, person B will probably have to reaffirm that they can meet up if the relationship between A and B is formal.
Another way to do it is to just say "I've made my decision." Translation using this structure below:
In reality, a lot of this depends on the relationship between A and B, as well as the environment where the conversation happens.