I think it perfectly illustrates his conniving and controlling nature. Asking why she wanted it was his way of saying no. He knew she couldn't and wouldn't justify wanting the 25 grand and even if she did he would have found an excuse not to give it to her.I think Ginger would subscribe to all of the conditions you have just laid out, but I'm not convinced Ace would. If it was Ace's understanding that Ginger was merely a 'trophy wife' and that it was his obligation to give her stacks of cash, no questions asked, then that scene would have obviously played out very differently. I'm not suggesting Ace was merely a meek victim in all of this, he was incredibly naive and ultimately deserved everything that came his way for refusing to turn his back on such a toxic mess, but Ace was controlling because he knew he could not trust Ginger. If you want to demonstrate Ace's controlling tendencies, you don't present a scene where he is merely asking for why his wife has just asked for 25 thousand dollars. That's the logical response. Why do you want the money?
She knew it was an utterly pointless endeavor to even attempt explain why she wanted it!!
It was Ace being a manipulative control freak while giving the ruse he's being fair and honest.
Just another example why I think the film is written and acted to perfection.