Change Shared file's permission from Edit to Read only

Hello, good afternoon.

We have a Site for the Finance team, one of my colleagues who is a Member of the site, shared a few documents with a few users outside the finance team (but withing the company)

to start with they were given Edit permission, but now that the files were revised accordingly, the permission should be changed from Edit to Read only so that they cannot keep changing the files.

My colleague R created the file, and shared it, but now, when he is trying to "manage access" they gets the following message (saying they are not the owner of the file - thought they created it) - and the same is happening with several other documents