IP address issues

[isaacl]

I run a software that is supposed to make requests to a remote computer, and the software is set up to only allow requests from a specific IP address, which in this case is set to my main server IP address.
For some reason though, requests are coming from one of the additional IP addresses that I have added in cPanel, even though that IP address seems to be set, and should be set, not to currently be in use.
The additional IP address block I'm trying to use are lower than the main server IP address (the new block is x.x.231.x, and the server IP address is x.x.235.x), which may be what's causing the issue.
I tried removing the additional IP addresses, and everything seemed to work, and then, after re-adding the additional IP addresses, outgoing connections still weren't using the main IP address (MySQL, etc), so I removed them again. It seems to always try to use the lowest IP address, which may be what's causing the issue, but the server IP is set to a different address than the one being used.
Can someone help me figure out what the problem is, and why it's using the additional IP address?
Thanks!

[cPanelTristan]

Is there a reason you cannot simply whitelist the other IP? It's likely due to network routing tables, but I'm not entirely understanding why the other IP cannot just be whitelisted.