I once checked this out and as it turns out there's quite a lot of cases where foreign aid is spent on weapons, even sometimes as a condition of foreign aid.
I was was stupefied that Belgium made a foreign aid donation to Saudi Arabia. I found the reason : it was contingent on the Saudis spending about 30x the amount of the donation on weapons made by a plant south of Brussels, with (I'm sure coincidentally) several (then) cabinet members on the board. They bought "weapons for crowd control" with it, because apparently that's what fully automatic machine guns are used for.
I think the issue is that weapons production is an industry that is entirely controlled by government action, both in the positive and negative direction.
Governments forbid certain parts of the market on one hand, and they cheat and lie and deceive everyone so their own weapons manufacturers would see more customers on the other hand. US, Israel, France, even tiny little Belgium all engage in this.