Set user role, if an specific role created an user

You can use user_register action, which is invoked after registering a new user.
To add or remove roles/caps, the WP_User class provides methods:

• add_cap(), add_role()
• remove_cap(), remove_role()

Your code might look like this:

add_action( 'user_register', 'se385135_user_register' );

function se385135_user_register( $user_id )
{
    $user = wp_get_current_user();
    if ( !isset( $user->ID ) || $user->ID == 0 )
        return;
    if ( !in_array( 'sales_agent', $user->roles ) )
        return;

    $created_user = get_user_by( 'ID', $user_id );
    if ( $created_user === false )
        return;

    $created_user->add_role( 'b2b' );
    $created_user->remove_role( 'customer' );
}