Inter are so tight to pass FFP, it would benefit them over next couple of years at around 10m pa. A rudimentary explanation is below and is due to the way player purchases and contracts are ammortised within the books of football clubs. Please note this is nothing to do with the flow of money which many fans confuse when looking at player transfers.
Essentially if they bought Lukaku for 5+5+60 this window, assuming a 5 year contract (at 10m pa), total cost of the deal would be 120m, impact to the P&L would be 24m per year over next 5 years.
By loaning for first 2 years, P&L impact over the next 2 years would be loan fee +Lukaku's wages (assuming they are paying all of these), would be (5+10), hence would make a P&L saving of around 9m pa over next couple of years compared to buying him outright.
This is a pretty significant saving over this period for a club struggling against FFP.
For United it would most largely have the exact opposite impact, however is less significant as United have no FFP issues. I think they would likely agree inprinciple however expect a premium on the loan fee or purchase price in order to agree.
Not sure if this makes sense, Im and accountant so better with numbers than words!