If your cPanel account is using a dedicated IP address, you will be able to send email from the dedicated IP address and have Apache use the dedicated IP for the site. However, from command-line or by script, you could not use 'sftp' with the dedicated IP address when making an outbound connection.
Below is an 'sftp' option that should help when making an outbound connection from a dedicated IP address.
Below is an example of the 'sftp' command that will send its outbound connection from a dedicated IP address on the cPanel server.
Example (sftp connection will use the 'dedicated' account IP):
sftp -o BindAddress=dedicated-IP firstname.lastname@example.org
Replace 'dedicated-IP' with the dedicated IP address of the cPanel account.
The option is shown on the 'sftp' manual page as well.