How do countries pay each other sums of money?

Yes, as I pointed out in the first response to the OP.

My bad. I skimmed over the thread.

True. But with weapon systems, the training of personnel might be done at a Government base, and the country will pay directly for that.

From a Forbes Article in 2008:

Great thread!

I asked the OP’s question in an Econ 101 class. I was an under grad freshman. It was 25 years ago. The instructor replied, “I don’t know, next question?”

Waited along time for these answers. Thanks all!