First play through with the Crew expansion and we had an occurrence of someone with a shortage, requiring them to move back to the first space they are able to pay the full cost.
They had a merchant whose ability (Canon<>Food) meant they could pay the cost 1 space back by exchanging canon for food. Does this ability have to be taken in consideration under shortage.
We played it did not have to be used and in the example above the person moved back two spaces to a treasure spot and got the +12 card.
I would assume not, as the card says 'may'...
The whole point of the merchant is to introduce more movement options, not fewer!