German scientists were concerned about Nazi restrictions on uranium sales because uranium would not be available for industrial research. uranium was important to German missile programs. uranium paid for German scientific research. uranium could be used in research on atomic weapons.

I believe the answer is: uranium could be used in research on atomic weapons

When a country restrict the sales for a certain resource, the most likely reason is that the country itself want to use that resource for internal reason.

German scientist aware that uranium is only useful to either create energy source, or mass destruction weapon.

Since they would most likely be notified if there is a new energy source is created, they are worried that it might be used for weapons.

They were concerned about Nazi restrictions on uranium sales because " uranium could be used in research on atomic weapons". Hitler was famously skeptical of the ability to build a nuclear weapon and the scientists disagreed.