I'd treat it as a two separate deals. We sold him last summer for 8m iirc, we had no ownership on player and used that money for other purposes, then decided to bring him this year for 18m. For instance we sold Vidal for 37m and if we bring Gotze for 40m, are we gonna claim that Gotze's cost was 3m?
18m for 4th/5th striker has very little sense imho, if there aren't some other deal involved, like Berardi. But that's just guessing.