Borussia Dortmund reportedly have concerns regarding the structure of Manchester United's proposal for Jadon Sancho.
Although Dortmund have frequently stressed that they have no intention of parting ways with the England international, United remain eager to negotiate a deal.
The Premier League giants are said to be willing to pay £90m for the attacker, providing that a portion of the fee is made up in add-ons.
According to ESPN, United officials want to insert a selection of bonuses which relate to the club's performances in the Champions League.
Dortmund are allegedly worried that United will not qualify for European football's premier competition every year, leaving them at risk of missing out on a considerable amount of money.
The Bundesliga outfit may remain against selling Sancho unless they receive a bid of £108m.