In summary: if he sells for less than the market price or less than the price stipulated for him, the sale is valid, but he is liable for the shortfall.

If he buys for more than the price stipulated for him or more than the market price, the purchase is valid, but he is liable for the excess.

This applies when the discrepancy concerns the amount. As for a discrepancy in the manner of payment—such as when he says, “Sell it for such-and-such a price on deferred payment,” but the agent sells it for immediate payment; or, “Buy it for such-and-such a price with immediate payment,” but the agent buys it on deferred payment—we say that it is valid, because there is neither an increase nor a decrease in this case. However, if this causes harm to the principal, it is not valid. Those qualified by experience determine whether harm exists. Thus, if they say that the purpose mentioned by the principal is valid, and that immediate payment or deferred payment entails harm, their assessment is acted upon.