Ethan, who is not very practiced at lying or deceiving people, doesn't want to take his wife to the station, because he wants to be at home where he can spend more time with Mattie during her absence. Taking her down to the station would ruin the chance he would have to spend at least one evening alone with Mattie; if Jotham takes Zeena, that would leave Ethan the freedom to have time with her, and no servant around to bother them. So, he conjures up some story about how he has to go into town and get money for the lumber. Originally, Jotham was going to take the lumber into town, but Ethan says that he wants to do it in order to get paid for it.
Unfortunately, he is not going to get paid for the lumber at all, and he knows that telling Zeena he will be will give her hope that
"he was in funds on the eve of one of her therapeutic excursions."
This would encourage her to buy expensive remedies that in reality, they couldn't afford. But, his desire to spend time alone with Mattie and to avoid Zeena overwhelms that caution, and he sticks to the lie. I hope that those thoughts helped; good luck!