If you've already dug up the Ancient Box, then you can give up. For some reason that sets the hammer_voucher flag, which makes it impossible to get that voucher.
Edit: Okay, I installed it on my phone and the save file's easily accessible, so here's how to fix the file. Go to Android/data/com.skullgear.games.dandy/files/ and copy your latest save to your computer (dandy_0.bin is the auto save). Then open the file with a hex editor (a normal text editor will corrupt the file) and search for the ascii string findable_voucher. Change one of the characters to something else (for example, findable_vouchee), save the file, then copy it back. Be sure you just change a character, adding or deleting any will corrupt the save. Doing that will make the voucher you found lying on the ground reappear, which lets you finish the event.
Edit 2: Started a new playthrough and it seems like it's explicitly digging it up in the graveyard that does it. Digging it up in the suburbs doesn't seem to set any weird variables. I'd still say your problem is probably the box's fault.