Ok I am gonna start as from the top.

1) On the new server create your suer without a default DB or any permissions to DBs accept those that already exist on the server beforehand.

2) Attach the DB you are copying over.

3) Open QA connected to the DB you just attached.

4) Run

EXEC sp_change_users_login @Action = 'Update_One'

, @UserNamePattern = 'LoginNameInTheAttachedDB'

, @LoginName = 'LoginNameToAssociateWithOnServer'

5) Correct default DB of the Login Account and any permissions that need to be fixed and maike sure can login and access the resources you need it to.