You could structure a loan and purchase to where it wouldn't be that prohibitive. If they loaned him, they're looking at 8-10 million a season. If you add that to a 55-60 purchase price, in installments, you can get close to the 85, without feeling like they're losing money.
8-10, obligation to buy if he hits reasonable targets, make the purchase price 60 on top of the initial 10, plus 10-15 in additives in reasonable incentives. Spread the purchase price out over three or four windows, and you're kicking enough money down the road to where it's more than doable.
Without getting into the minutiae too deeply, it doesn't quite work like that. There's the actual cash (such as spread payments), but there's also the football accounting which is abiding by FFP and that doesn't necessarily align with the real cash changing hands when it does.
Essentially, it gives them the flexibility to postpone payments (outside of wages+loan fee) and push it into the next fiscal cycle after they've accrued more revenue and wiped the slate clean. Inter has been working with this model since the new ownership has been in place. And if there's an option on the loan, well, the benefits are clear as day. You ostensibly get a free look at a guy.
Furthermore, if you add players for a swap deal, to "lower the cost", the delineation of value changes completely. Again, another trick Inter has been using to get around FFP. See Zaniolo+Santon for Radja. If they were to acquire Lukaku permanently, it wouldn't surprise me in the least if they tried to move a couple of prospects to United (Irish CB Ryan Nolan comes to mind).
In the last few seasons they've done loans with options/obligations with:
Keita Balde
Politano
Cedric
Vrsaljko
Gagliardini
And those are just the ones I can think of off the top of my head. Some of which they're obligated to begin payments on this summer. It's not an uncommon practice in Italy at all. For those that aren't aware Inter just recently got out of their own problems with FFP and UEFA, as seen by the roster restriction penalty they received in European comps this past season. They are by no means in smooth sailing territory yet.
An Icardi sale would definitely change things for them, but I don't think it means they'll drop 85m on a single player, and certainly not for Lukaku.