To restore faith in the 737 MAX, Boeing needs to prove its flagship jet is not just airworthy but also a safe investment.

At a gathering in Dublin this week of the titans of the multibillion-dollar aircraft leasing industry, which finances half the world’s fleet, cracks were appearing in that effort.

Boeing said on Tuesday its troubled workhorse – grounded last March after two crashes in which 346 people died – should receive approval by mid-year from US regulators, paving the way for hundreds of jets to resume service later in 2020.

But in scores of high-stakes negotiations in the background, it is trying to convince banks, leasing firms and airlines that the investment case for thousands more of the jets – worth hundreds of billions of dollars – remains intact.

Airplane owners and investors said some lenders were already demanding higher collateral in deals on the MAX. One airline said financing for pre-delivery payments had dried up amid the uncertainty – though the market won’t be fully tested until closer to renewed deliveries.

“Even people who have committed to financing previously are wondering should I extend or should I just pull back to wait to see because they don’t know the real value of their collateral going forward,” said the head of an asset-management firm active in the sector, declining to be named to preserve relations with Boeing.