It's two months later. Kate comes to visit Merton in his room. This happens shortly after an official letter from New York has arrived to tell him about his inheritance from Milly.
Aunt Maud, in the meantime, has tolerated Kate's stay at Marian's for two months. She's willing to let the family drama run its course, since she's confident that Kate will come back to her.
Of course, Merton now finds that he's always welcome at Lancaster Gate, since he's now a very rich man.
It turns out that Merton is feeling bad about Milly sending him her money, since he and Kate pretty much swindled her. So he gets an interesting idea into his head. He gives Kate an ultimatum: either marry me with no money at all, or reject me and I'll give you all of Milly's money.
In other words, Merton needs to know that Kate would marry him without money. She can either have his fortune or she can have him, but not both.
Kate says she'll marry him for just himself if he promises that he's not in love with the memory of Milly Theale. Merton says he just wants to marry Kate and go back to he way they were before Milly came into their lives.
Kate closes the book by saying that they'll never again be as they once were. It's pretty ambiguous, and we're not really sure what her answer is to Merton's ultimatum.
A total bummer for anyone who's looking for closure.