Prevent SharePoint user uploading files but still allow to share externally

[shamjamf]

We have a SharePoint Document Library which holds software updates for customers. We would like to control who can upload new files to this library by preventing our support engineers from uploading files but would like them to have the ability to share existing files externally.

If I make the support engineers read only, I can't find a way to give them permission to share externally.

If I make the engineers 'Members' who have edit permission, they can share externally but they then have the ability to upload files.

I can modify the view that members see to hide the upload button, but they still have the ability to Right Mouse Click and choose to upload.

How can we allow our engineers to share files externally while preventing them uploading new files?

[#2]

@shamjamf 

You can create custom permission groups for this, here you can add granual control of what permisisons to allow

To create custom permission:
https://Site.sharepoint.com/sites/yoursite/_layouts/15/addrole.aspx

Create group based on that permission:
https://Site.sharepoint.com/sites/yoursite/_layouts/15/user.aspx

Break the Library permisison after that and remove the groups member and add your custom group

Remember to add the users to this group

[#3]

@NicolasKheirallah Thank you for your reply, however, unless the custom permissions allow full edit permission, the ability to share externally is no longer allowed.