Has anyone tried to enter the US with a firearm, let alone one with the serial number wrong?
In Calgary, we pre-clear US customs at the airport here, and even with the proper ATF forms pre-approved, it can take up to an hour (though usually half that long) for US CBP to let you go with the rifle. If a serial number is wrong, you can generally go anywhere - except the US. If you didn’t pre-clear in a foreign country and were to show up in the US with a firearm and without a valid permit from ATF as a result of a wrong serial number, you would be a non-resident alien unlawfully in possession of a firearm, and that’s a bad situation.
I should say though, that if you arrive in Canada with a properly declared “regular” rifle (not an AR-15 type), and you have a wrong serial number, they will fix that up at customs. It will take some time, but you aren’t going to jail.
As for day rates in Zimbabwe, outfitters generally are restricted to the government allocated quota for animals in their concessions, particularly dangerous game (a Zim operator could tell us if that applies to all game and to campfire areas as well). So, for example, an outfitter could have 2 lions on quota (as one concession I know does). Each of those hunts requires a certain number of days to be booked, and the higher daily rate for dangerous game be paid. If you shoot the dangerous game, or the one animal you want, early in the hunt, you can’t expect to have the hunt revert to a lower plains game daily rate for the balance, or to get a refund of the daily rates if you choose to leave early.
The daily rates are a huge part of how the outfitters make their money, given that they don’t own the animals (as they generally do in RSA) and are paying a substantial part of the trophy fee to the government.