Why would PSG sell Zlatan to us for 15 millions? He's their most important player and cost them 20 millions one summer ago. If they're letting him go (hard to imagine, unless he forces his way out), I doubt there's going to be charity involved. So lets see, 4x7 is 28, plus taxes that's 50 million, plus transfer money it's 70 million.
From a financial point of view, paying 40 millions (which our club almost never does) for an u25 player would probably turn out cheaper than signing an old Zlatan here.