The Serie A side made a bid of €60m (£54m) for the Belgian forward, but United want to at least make back the money they spent on Lukaku in July 2017, when they paid Everton £75m for his services.
Inter manager Antonio Conte is desperate to sign the 26-year-old, making him his primary transfer target when he took over at the club this summer, but a deal is proving very difficult to be done.
Marotta believes that the offer Inter made to United is a fair one, and does not feel that the club needs to pay over the odds for the former Chelsea man.
‘The club is strong, but we need to stay within certain parameters,’ Marotta said. ‘The club has made big investments and we believe that we’re already at a good stage in the process.
‘We are ready to pounce on opportunities, but prices are nowadays very high indeed. We made a strong offer, one that is valid and suited to the transfer value of the player and we were unable to conclude a deal.
‘We’ll continue to monitor the situation and anything can happen on the market.’
A move to Inter looked certain for Lukaku this summer, with no other club showing an interest and Ole Gunnar Solskjaer leaving the Belgian out of pre-season fixtures.
However, Juventus have now emerged as another possible destination for the Belgian, having entered the race for his signature.
Reports suggest that the Serie A champions are looking into a swap deal for Argentine striker Paulo Dybala.
While Lukaku would be keen on the switch to Turin, it is yet to be seen if United can convince Dybala to move to Old Trafford, with Italian publication TuttoSport reporting that the striker would insist on a big pay rise to complete the move.
Dybala is paid €7.5m (£6.87m)-per-year at Juve, although this is less than Lukaku earns, so a straight swap should be possible.