Microsoft is offering $100 off on its premium 2-in-1 Surface Book, but the offer is only valid for models running on the Intel Core i5 processor.

First spotted by Neowi, the promotional deal is exclusively meant for customers in the US (including Puerto Rico). It will be true through to April 9 in the US. The Microsoft Surface Book with Intel Core i5 processor onboard comes with two internal storage options - 128GB and 256GB.

The Microsoft Store is now selling the 128GB Surface Books with 8GB of RAM at US$1,399 (AU$1850), down from its original price of US$1,499. The other model with similar RAM and 256GB of native storage can be purchased at US$1,799 (AU$2379).

However, the higher end models with the 6th Generation Intel Core i7 processor are not included in the promotional offer.

Running on Windows 10 Pro, the Microsoft Surface Book features a 13.5-inch PixelSense touchscreen display, which can be easily detached from the keyboard. The 2-in-1 device is supported by Surface Pen.

With every Surface Book purchase, Microsoft is also offering $30 (AU$40) off on the Surface Dock.

In related news, Best Buy has also slashed the price of its retailing Microsoft Surface Pro 4 by $100. Both the 128GB and 256GB storage models, powered by Intel Core i5, are on sale.

The base model of the Surface Pro 4 with 128GB of native storage on Best Buy costs US$899.99 (AU$1190), whereas the 256GB variant is priced at US$1,199.99 (AU$1586). The Microsoft Surface Pro 4 touts a 12.3-inch touch-screen display with a resolution of 2736 x 1824 pixels.

Microsoft is expected to refresh both its Surface Book and Surface Pro 4 line-up this year. According to prevailing rumours, the highly anticipated Surface Book 2 and Surface Pro 5 will be unveiled at the same time as the Surface Phone.

However, Microsoft is yet to announce the official Surface Book 2 and Surface Pro 5 release date.

Related: Microsoft Surface Pro 5, Surface Book 2 release date update