Cannot use special principal dbo: Error 15405

I am trying to give all the permissions to a user in the User Mapping section of a database. But, I am encountering this error:

"Cannot use special principal dbo"

Server roles of the user:

This is happening because the user 'sarin' is the actual owner of the database "dbemployee" - as such, they can only have db_owner, and cannot be assigned any further database roles.

Nor do they need to be. If they're the DB owner, they already have permission to do anything they want to within this database.

(To see the owner of the database, open the properties of the database. The Owner is listed on the general tab).

To change the owner of the database, you can use sp_changedbowner or ALTER AUTHORIZATION (the latter being apparently the preferred way for future development, but since this kind of thing tends to be a one off...)

>Fix: Cannot use the special principal ‘sa’. Microsoft SQL Server, Error: 15405

When importing a database in your SQL instance you would find yourself with Cannot use the special principal 'sa'. Microsoft SQL Server, Error: 15405 popping out when setting the sa user as the DBO of the database. To fix this, Open SQL Management Studio and Click New Query. Type:

USE mydatabase
exec sp_changedbowner 'sa', 'true'

Close the new query and after viewing the security of the sa, you will find that that sa is the DBO of the database. (14444)

Source: http://www.noelpulis.com/fix-cannot-use-the-special-principal-sa-microsoft-sql-server-error-15405/

This answer doesn't help for SQL databases where SharePoint is connected. db_securityadmin is required for the configuration databases. In order to add db_securityadmin, you will need to change the owner of the database to an administrative account. You can use that account just for dbo roles.