In the previous post, I noted that Woods asked the question of why it was immoral to double pay for something. This question is strange in the absurd, because it is nearly axiomatic that an exchange is said to be just when what is exchanged is equal and double paying violates that equality. Woods stipulated that the double paying be consented to, but this is only another absurdity of supposing mere consent makes something good or just (the case of prostitution being the paradigm of a counterexample).
However, in spite of Woods, let us examine what an exchange is and how it is just. An exchange is in interaction wherein something is given for something else. This differs from a gift where something is given simply; the action is one-sided. This is also not a sort of double gift where one party gives something and the other party gives something after which both have given and received. In the exchange, the giving and receiving are mutual dependent. I am giving in order to receive something and I am receiving something because I have given something. Whereas with a gift, the reason for the gift is extrinsic to the gift, perhaps meeting the obligation to a brother or the perfection of the virtue of liberality.
Now in an exchange what is owed for what is given? Just as form must be proportionate to matter and the giving is as matter to the receiving, so that which is received must be proportionate to what is given. There is then a certain equality in the exchange. This is what constitutes the justice of an exchange, that one gives proportionately to what he receives.
Let us consider some example. What reason could I give were I to give you more than the proportion owed you? Indeed, it seems that this reason must be extrinsic to the exchange itself for the exchange provides only reason for an equality of the exchange. The reason I may give is out of certain gratuity as when in “fair trade” practices, we pay more than market price for something. After going to dinner, I may add a gratuity for the service, not because it is owed on the account of the exchange, but in virtue of local custom or special praise for good work or some other reason. Note though that the reason is extrinsic to the exchange and the exchange itself does not account for the excess but only the equality.
The “double” paying that Aquinas speaks of is indeed unjust in itself, though it may be justified by extrinsic reasons.