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I just had a look and tried everything I could. I'm assuming that if you were to put on an anti-sale you'd have to set the discount figure to a minus number (so if you wanted to double the price, set the discount to -100%) however because this works on a slider system which starts at zero and not a type in system, such a "discount" isn't possible and even if it was it'd likely be coded so that you couldn't input a negative value. (An anti-sale is something I've never encountered and is completely new to me.)

I'd say the best way to achieve this would be to do what SolarLune suggested and raise the minimum price temporarily then lower it again when the anti-sale is over.

I have to ask though just out of curiosity, why would you want to do this?