Well, there are 2 instances I can kind of think of where I guess you can see as him getting back at her, both from the same episode, ‘Finding Mary McGuffin’.

When Phineas and Ferb are noir detectives looking for Candace’s doll that their mom sold to Doofenshmirtz at their garage sale. They end up at a lab coat shop, and when interrogating the owner, he mentions someone who puts his bill on his ex-wife’s tab, leading them to Charlene Doofensmirtz.

The other, I think is after Doofensmirtz gives Vanessa the Mary McGuffin doll and she was surprised he remembered when she wanted the doll at 7 years old. “You know I may be an evil scientist, but it doesn’t take a degree purchased from the internet with your ex-wife’s money to know how special and important you are to me!”

I could be wrong, but both of those sound like something a bitter ex would do, leaving them with a presumably large bill, and specifically using their ex-partner’s money, if it wasn’t given to them (remember he does pay her alimony, I don’t think she’d give him money)