We'll redeem Immobile from Torino for say 9-10m,which means he'll be fully owned by us.We'll then sell him to Dortmund for 19.5m euros which then can be reinvested into a new player.We should not look at how much we gain from the deal but how much we have in hand i.e 19.5m euros once he's sold to BVB.
Like somebody said,the difference between selling and not selling Immobile is 20m.