Hey Hawawaa, thanks for the bug report.  When you click on those credit entries, a popup is supposed to come up allowing you to choose how many credits you are dealing with.  Have you been able to get this popup to ever come up without a crash?  For example, if you select your own credits for offer, or try to set up per-week payments?  If you cannot, try verifying your files through Steam (right-click the game in your properties, select "Properties," go to the "Local Files" tab, and select "Verify Integrity of Game Cache"), then try to trade credits again.

I hope it is as simple as that screen being messed-up in some way, but let me know if that still doesn't work, or if you are able to see the screen in some situations but it still crashes in the scenario you specified above.

I click offer credits and the game crashes.

Dropbox- https://www.dropbox.com/s/wfke6uz8gc0if8z/GalCiv3%20Crash.7z?dl=0

I'll start a new game.

Was this after today's update?

Thanks for the update; the large UI is the key, it is what's causing the crash you are getting.  To help you be able to trade credits, there are two options.  

I'll attach an updated version of the screen file that is causing the trouble.  You can use that file and replace the one you find in Steam/SteamApps/common/Galactic Civilizations III/Screens with it, then go to common/Galactic Civilizations III/Tools and double-click the "ScreenDataCompiler.bat" to update your screen data.  This way you can keep running the game with large UI enabled, and you should be able to pop up that popup without it crashing anymore.  As a note, though, if you verify your Steam files or otherwise reinstall or update it, you will have to follow these steps again because the changed file will be overridden.  This will be fine when our next patch comes out, because it will include this fix anyways.

Updated screen:

https://www.dropbox.com/s/iyyi3k7oj4tl340/DiplomacyQuantityWnd.xml?dl=1

Alternatively, the crash does not occur if you run with small UI instead of large UI.  Doing this in general may also fix some places where our large UI does not work or look its best yet that you may have noticed already.  If you have it on large UI because of a very high res screen or the text otherwise being too small, you can try running in lower resolution and scale things up that way, if you don't mind the blurrier map.

Sorry you ran into this issue, hopefully one of these solutions helps you until the next patch comes out.