The 2020 iMac screen's part number is 661-16087 (non-nano glass) or 661-16088 (nano-textured glass); the 2019 uses part number 661-12558. I doubt they're interchangeable, especially since (alert: bigger problem ahead) replacing the screen on a 2020 iMac requires the use of Apple's System Configuration utility before the display will work. So even if the 2019 screen would physically and electronically fit the 2020, it won't work until an Apple Store or an AASP runs System Configuration on your iMac after the repair is complete, and they can't do that unless they also process and complete the repair. (Welcome to the world of Apple software locks.)
You're not going to find a "cheap" display for that 2020 iMac. A new part from Apple is $660 (non-nano) or $1100 (nano) and the machine is only two years old.
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