If a user is granted write permissions to a directory, the user has the privilege to manage permissions of every entry in that directory. This privilege includes the assignment of read-, write- and execute permissions on every file and directory contained in that directory.
By extension, the user could also gain every permission to every file inside a sub-directory: The user could assign write permissions for the sub-directory and then proceed to manage the permissions of the files contained therein. This means that if a user has effective write permissions on a directory, that user also has potential read-, write- and execute-permissions on every file and directory in that directory and any of its sub-directories. Those permissions might not be immediately effective, but the user can gain them at any desired time.