This is an issue that has been raised in the past.  The timing you sign this contract makes the whole administration of this tax one fine mess. 

Laws are supposedly to be applied equally and without prejuidice.  This WFT is no law (not by legal definition anyway) it is a contract with the government.  As a result, you take your chances in fixing a price, you can win or lose on that basis. 

I wonder if the effect of this tax could be hedged away?  You can hedge foreign currency exposures, why not hedge this tax as well.  All you need is a willing counter party and pay a fee. 

Just a thought.