Enable editing docs but restrict uploading new docs - SP 2013

Question

Im using SharePoint 2013 foundation. The site level users are stored in a custom sql datatable. Users can download documents in a document library via a web part link. (SPGridView link)

Users should be only allowed to edit available documents and save back. Editing existing documents through client application(Eg: Microsoft Word) and saving back works !

But the issue is when user creates a new doc ( for instance creating a copy in Microsoft Word) and trys to save back via words menu:

File > Save & Send > Save to sharepoint > and then chooses a library.

Lets say user downloads/opens a file in client application(In my case Word/Excel/PPT). Does some changes, then renames the file and attempts to send back to the document library through save and send, as below:

Can this be stopped ? Basically I want users to edit existing documents in the library but not upload new ones !

Answer 1

You need to customize the permission level for that document library, and allow users to Edit, but not to Add.

Answer 2

There are two approaches: in the first approach, you make a new permission level,where users can edit but not delete or add = contribute minus these two :

http://technet.microsoft.com/en-us/library/cc288074(v=office.14).aspx

Second appoach is to set the list readonly for every user. Whenever an item is inserted to the list, a Sharepoint Designer workflow will add the contribute permission to that list.