The Nerazzurri are determined to get a deal over the line for the Belgian international, who has not travelled with the rest of the United squad for a pre-season trip to Norway, but at present the valuation of the 26-year-old has been a sticking point.
Marotta has claimed Inter have already had a substantial cash deal turned down by the Red Devils although he insists the Serie A outfit have not given up hope of securing Lukaku's services before the end of the summer transfer window.
"We made an important bid to Man United for Lukaku (£54m) but Man United ask so much for Romelu," Marotta said. "We didn't find an agreement but we're still working for Lukaku. I don't know what will happen on next days, football is so strange. We have made a fair and important offer respecting the value of the player."