Folder permission

I have an Windows 2008 R2 file server with clients on Win 7 & 8, there is a requirement for 2 different departments to have a common shared folder with different permissions asper the below scenario, 

Common shared root folder with 2 sub folders (example sales, purchase), now we need to share the root folder and map the drive, however sales users should have full control to sales folder and read access to purchase folder.Also the vice versa..purchase to have read access to sales folder & full access to sales folder.

Howe to achieve this...??

I have shared the root folder and granted permission, however both users can either have access or denied while trying to create a file or folder.

Thanks.