A well-regarded development organization I used to work for went through a quiet reorganization in 2016 when a few million dollars came up missing. It turned out that just getting materials to rural places in Sub-Saharan Africa was really, really hard to do without graft along the way.
I don't know how things ultimately resolved, but it's not obvious to me that effective countermeasures would have been cheaper than letting some of the money leak out in the delivery chain. It's an inherently hard problem to disburse development aid to the right parties in the right amounts.