Fellow Allies!
(You know who you are ... even if I don't!

)
Our Minister of Finance has come to Us with some disturbing news - apparently Our People think that Our Free Trade policies have resulted in an unfair Balance Of Trade. For example, Germany can send $50 or more to Us, while We can only send $5 to Them. The trade unions here are complaining!
We were able to fend off the trade unions when the World War was hot, by our Generals explaining that Germany, Russia, France were only surviving the huge and well-financed British and Norwegian onslaught by reason of this African Trade, so the trade unions could just suck it up.
But now those Generals have been executed (see above a few posts), and the World War may not be a World War any more, if King Matty's suspicions are correct. Also, due to Our (deceased) Generals' incompetence, We desperately need money to rebuild our ground forces - We now have less armies facing the British than ... well ... Germany. Ahem.
So We are going to institute a Tax on Unfair Trade Balance: $1 on every $5 traded to Us in excess of what We can trade to that Trading Partner.
What does this mean?
Well, for Germany, for example, if They send $5 to Us, just as much as We can send to Them, there is no tax - they get $10 back.
If They send $10 to us, there is a $1 tax - $1 is taxed, $9 is traded, and They get $18 back. Still a very profitable trade for Them, of course.
If they send $50 to us, there is a $9 tax - $9 is taxed, $41 is traded, They get $82 back.
Makes sense? So the idea is that trade is still almost as profitable for our Allies, just that now We Africans get something back from this as well. And this gives our Allies some incentive to build up their provinces - every $1 they build up isn't just $1 for the Ally, but an extra $1 that they won't be taxed for Unfair Trade Balance.
Oh, and of course We will be very willing to pay Trade Balance Tax if one of Our Trading Partners grows large enough to become a recipient of more of Our trade than They can send to Us. Fair is fair, after all.