Contracts profitability should consider exports such as slag as the value to maximize.
Currently, when you use the offices to increase contract profitability, it reduces the amount of slag that gets exported and increases the amount of sour water you get in return. But this contract obviously is a "exchange landfill for another environmental problem" you need to deal with. I don't want the offices to increase the amount of sour water I get. I would like to increase the amount of slag the boat takes from me, and reduce the amount of sour water I get.
I would either suggest that you could trigger wether offices should put in some elbow grease to increase or decrease a certain side of a contract, or just leave it alone. Maybe, instead of a flat bonus, you could spend the government points directly in the trade harbor boosting a certain contract, and on what side. This gives you the most flexibility. I don't really need to boost my trade for limestone, but I'd love to get more bang for my buck trading iron ore.