On the scale- We're talking about millions of checks a year. You've effectively proposed to ask every congressperson to spend all day signing checks. By doing so, you've also eliminated the time they spend working with constituents on issues, understanding the facts or background of decisions they've made, or even working to find compromises.
On the assumption- There isn't a dollar figure, but there are quite thorough rules. (https://crsreports.congress.gov/product/pdf/R/R46497) This spells out how the rules are established and what governs it. You can quite easily look up the authorizing legislation for USAID and see the allowed purposes for funds. Definitionally- that makes it not slush funding.
It's the standard you've asked others to be held to in these comments. It's fair to be held to it yourself.
All the relevant documents I should be able to link to have been purged from various Federal sites due to “DOGE” shenanigans.
Congressional appropriations are how money is allocated, regardless of party in charge of Congress or the Executive. If the money is misspent there’s a range of tools available to Congress AND the Executive to correct the problem. But if we're just going to let a group of people decide on their own what is or isn't fraud then, regardless of your political belief system, we're simply fucked.
Heck, it would at least give them something to do, and feel the money roll and make their choices in Congress mean something again.