When selling cotton to an external market the scale (if there's any left after turning the card) provides what the rules describe as "an additional amount" of between £1 to £4.
It's ambiguous whether that extra amount is:
An amount in cash, as a one-off payment from the bank. The amounts seem negligible if this is the case.
An additional income. The point of doing this action is to increase income, so the word additional suggests this is additional income.
If it is additional income, is it an increase income squares or income bands? The scale is marked in £ so that implies an increment of a salary band.
Various other rule summaries and versions seem to just echo the words of the original, so they're not much help.
We've played both #1 and #2 without much impact either way, but still very uncertain which way this is supposed to be. For info, I've a first edition copy of the game, so this may be clearer in later editions.