My two pence worth regarding any deal and the reasoning behind it would be...
We offer Isco a contract saying we will defo use him more than Madrid have for this second half of the season. However the main reason we are getting you Mr Isco is because Pep wants you.
It just so happens there is a situation to exploit here for your and our benefit. That being Real are in the middle of a fight on many fronts, you are rightly a bit unhappy. They may want rid off you just so they can focus on other things.
This helps with messy negotiations possibly if we make a fair offer, they may just accept and wash their hands of this business. That leaves you with us ready for our main attack on Europe and the footballing dynasties. They have their recent and long past glories, we have some but have it all set up for much more new success. You are coming into your prime just as we are ready to stake our claim as European top dogs. We will have many years of success here with you part of that.
This sounds perfectly reasonable as an offer imo.