No. That's the way the official cards are worded, too.
But on those cards you first buy something and then you do something else. Like when you buy victory card gain gold. You first pay, then buy victory card and then you gain gold.
You now want to buy something, then reveal something then go back and during that buy do someting - overpay.
At least as I understand it overpay occurs during the buy not after buy.
If my thinking is correct, then you want to interupt that buy like Trader interupts gain. And Trader is worded "would gain"
Maybe the problem is with my understanding of english (I'm not a native speaker)
If someone would told me: When you buy a bread jump. I will first buy a bread and then jump, not during the act of buying - is that corret?