Use the ParseDN expression to choose what placeholder you want to pull the value from.
When pulling the manager field through LDAP it contains extra text rather than just the manager name.
If you use the 'Preview Data Import', it will look similar to this:
value "CN containers=FRS CN=Services...OU= Tom Turkey
The expression you would use is the ParseDN expression. Attached is documentation on how to use it, it will basically grab a distinguished name depending on the position of the name you want to use.