I have a small task that needs to be done and I projected to let the client pay 400 €. However, he now asked that the deadline should be set to next Saturday (in three days), and proposed to give 300 € if I do the task and 400 € if I do it for Saturday. 

I don't think I would drop my rate to 300 € because it sounds a lot devaluating, so I would like to let him pay 400 € if I do this task at all and 700 € if I do it for Saturday. Does that sound okay to you (Friday is a holiday here in Europe, so I officially only have one day to do the task, and especially he only has one day to find another freelancer, which I doubt he could do because he already builds the whole site with my contract).

By how much can you increase your rate if the work is urgent?