The Brute wrote:
After buying goods, you may return one other good on that island to the supply
As written, I could see how it could be interpreted to mean if you're buying from two islands then you could put back one from each. However, my intuition is that's not what was intended, and it's meant to be read more like this:
"After buying goods, you may return one other good on an island you bought goods from to the supply."