The Italian club's opening offer of €70 million paid over two years has been rejected by United, according to the Guardian's Fabrizio Romano.
Inter sporting director Piero Ausilio was in London on Thursday to begin discussions over a potential move for Lukaku. Although it was no surprise a deal wasn't concluded in the first day of negotiations, United's €83.5 million asking price could prove to be a huge obstacle for Inter.
Inter were already looking to stagger payments in their €70-million approach. They apparently offered €10 million to take Lukaku on an initial two-year loan, followed by an obligatory permanent transfer after installments of €30 million paid in both 2020 and 2021.
Nerazzurri boss Antonio Conte is understood to be keen on signing the Belgian frontman, but Inter have already spent around €61 million this summer and are still looking to raise funds by offloading unwanted pair Mauro Icardi and Radja Nainggolan.