According to a bunch or international law girls, international law could be the right tool to fight diversion of foreign aid into African presidents' bank acccounts. An exmaple is Omar Bongo, President of Gabon since 1967 who owns 39 properties, 70 bank accounts, and nearly 1.5 million euros worth of luxury vehicles. In January Bongo, the world's longest-serving ruler, was publicly criticized by members of the U.S. Congress for having arrested and otherwise harassed members of civil society who campaign against public corruption. (They give other fascinating examples)
Here is how it would work for France, for example. The french government gives aid money to Gabon. Bongo takes his share and so do French foreign aid workers. But international lawyers (working for the french government) want their share too so they start investigating the case but will want more rents so will always have an interest in foreign aid corruption. The business of aid continues, everybody gets his share, except African entrepreneurs. Perfect!
Giving foreign aid to corrupt government is a bad idea, end of story.