What an ignorant question. You can bet if I promised you a case of your prozac in exchange for an amount equal to your welfare or disability check and you didn't pay me, I would sue. A contract is actually PART of the entire trade process. Are you really the best your side has?
Whatever you feel it is, you were still unable to understand it or answer it. But that's OK, it is perfectly normal for those who who don't have answers to lash out with insults.
Let's say that we had your hypothetical Prozac/welfare check contract, and I didn't pay you. Isn't that indicative of the true problem in free trade? Apparently you are unable to recognize that it is a moral failure that causes breakdowns in trade. If I paid you the welfare check, and in turn you withheld the Prozac, then one needs to wonder why you did. If you supplied the Prozac without taking payment at the time of the trade, and I took advantage of your naivete by withholding payment then I would have an advantage in that I would have the Prozac and use of the welfare check.
This is typical of every breakdown in free trade. One side is trying to get more out of the deal than the other. Since you are completely unable to see this, please meet me with a case of Prozac, and let me prey upon your gullibility. I will bring the title to the Brooklyn Bridge in fair trade.