Why would Sanchez go to United?
Arsenal will take any offers and if United have offered more then fair enough they will accept that.
It still does not mean Sanchez has to accept that, in the summer I reckon he had offers from PSG and Bayern but he chose to wait for us.
Are us, PSG and Bayern poorer options than United? No, he will either wait until the summer and cost us nothing or Arsenal will crumble and sell him to us this month.
Also remember, United offering more means he will cost us more, it is a no brainer move for what is a very cheap player now.