Maran Yoseph Karo z"l states in the Shulhan 'Arukh that just thinking about one's business on Shabbath is permitted. However, because of 'Onegh Shabbath (the commandment to delight in Shabbath) it is a Miswah (Mitzvah) not to think about one's business at all on Shabbath. In fact, one should consider all of one's work to be already completed.
Hakham Yoseph Hayim, 'a"h, mentions in Ben Ish Hai that one who thinks on Shabbath about work he intends to do during the week will find that his work will not be blessed. This, he adds, is because he thought about it on Shabbath.
(See Shulhan 'Arukh 306: 8. Ben Ish Hai, 2nd year, Parashath Wayyishlah, Oth 4)
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