The bit I know is that USAID doles out significant piles of money to people who are supposed to spend it on certain things and not others. They hire companies to audit the expenditures. Those companies hire accountants who are not very talented. Those accountants make competitive salaries, but get very high bonuses when “things go well”.
USAID is only as honest as those who are in charge at US Missions abroad. Having said that, the organization is corrupted by those who use their positions for personal comfort, both with housing perks and TDY monies for travel. Once someone is at the Grade of FS2/1 or GS14/15 they can manage to take official travel to the tune of 50-60K annually and be off from work for official duties elsewhere about 30-45 days a year, then add on the annual leave used in conjunction and you get the drift. As for the doling out of Official Funds for program expenditures, think kickbacks, personal favors, etc. Our entire US Dept of State and USAID, along with other agencies seem to reward those who are at the top of each organization along with those they work with. As someone who served as both a Certifying Officer and Contracting Officer, I found it very difficult to question expenditures when dealing with this very management.