The author of Hebrews is quoting Habakkuk 2:4 from the Septuagint (as opposed to the Hebrew.)
In the Hebrew, this part of the verse would literally translate something like this:
"Behold the scornful; his mind shall not be happy" (Stuart)
(Part of the difficulty in translating Heb. 10:38 is that this is an English translation of a Greek interpretation of a Greek translation of a Hebrew writing. Meanwhile, Hab. 2:4 is an English translation of a "modern" Hebrew interpretation of an ancient Hebrew writing. This makes direct word-for-word comparison of the English very difficult.)
It is important to remember that the New Testament writers were not dogmatic adherents to our present Hermeneutical models! Their heremenutic went something like this:
1) Understand the Scriptures well enough to explain the meaning
2) Teach from a convenient translation (e.g. Septuagint)
3) Relate the meaning of the passage being "cited" (as opposed to a literal word-for-word translation)
4) Paste together whatever Old Testament passages seem necessary for supporting the argument
In other words, they "quoted" Scripture a lot like we do today in everyday conversation: close enough to serve the intent of the teacher. In contrast, modern Bible scholars are very concerned with exact literal word-for-word translations and careful consideration of history, grammar, immediate local application, etc. Since we think differently about interpretation today than they did in the Apostles' times, we are often confused by their methods of quotation.
At the end of the day we can rest assured that all Scripture is inspired by God and profitable for teaching, reproof, etc. Therefore, we can assume that between Hab. 2:4 and Heb. 10:38, we can look at the Hebrew and Greek that we have and get a sense of what the true meaning is.
And in this case, as usual, you could read it either way it would still be true (we know this based on other passages.)