Schäfer, 5th ed., gives Abrechnungsstelle. However, in the context of IT, Back-Office (with or without the hyphen) appears to be the prevalent usage. I can't cite any specific instances (sorry), but I've seen it often enough in various German texts.

My, my, our Billy will be well displeased that his products are so unknown ;-). "Back Office" is the name for the technology that Micro$oft use to connect and coordinate their "Office" products, especially when used in a network enviroment. Basically its a set of tools and interfaces which are common toall the "Office" products. As such it is, of course, a product name and therefore untranslateable. If you ever see a reference to "Back Orifice" then this was a set of hacker-tools that appeared a couple of years ago and, while they offered users access to hidden functions in office, they also short-circuited the security measures in various Office programs and allowed hackers to break into systems

"Microsoft BackOffice Server" is the name of the Micro$oft platform / platform program for coordinating many tool/connection programs, such as SQL Server, etc. See: http://www.microsoft.com/GERMANY/msdn/BackOfficeDev/default.... or http://www.optimal.de/. There are also "back office" (lower case, hyphenated or two words, adjective) platforms / platform programs from other software houses that do the same thing. The whole idea comes from the English expressions "front office" and "back office" which are older than computers. Ages ago when I was young, the "front office" staff were the employees who actually had contact with the customers. The "back office" employees were the bookkeepers, accountants, stock clerks, etc whose support work for the front office employees was absolutely essential but often unrecognized, unseen and unappreciated. "technische Abwicklung" or "Koordinationssystem" or "Hintergrundressourcensystem" would be possible German translations but "Back-Office" has become so "eingedeutscht" that I would not translate it, just capitalize both words and use a hyphen.