That would cause compliance issues on steam though publisher wise. They would need the title to be off steam since one of steams publisher terms is that the sale price of an item must be at least equal to the lowest price available on other platforms. Meaning that if you have a deal like described there, steam would be the higher price and it would violate their publisher terms.
edit: looking into this it looks like it might only be for steam keys, so actually they may be in the clear here.