Rules about giveaways with invalid keys you don't expect (and have no money to buy another one)?

I don't think public giveaways are allowed custom rules, but you can make a whitelist giveaway with such a condition that the winner will allow deleting the giveaway if it turns out to be a bad key. You'll still lose a giveaway slot though, I think.

The descriptions you see are not binding - if the winner doesn't want to allow a deletion they can just wait the seven days and click not received if the person making the giveaway doesn't provide a working valid steam key for the exact game listed.

But a lot of people are nice enough to allow a deletion in case of problems - you are more likely to find these nice people in the forums in unlisted giveaways where you are clear about the problem up front.

No guarantees in other words, but you can make it more likely by being nice yourself. :)

You might also take a look at Disclaimer in a GA and Is it really ok to do this?. If you are not sure if your key is valid you should either use Orphan Keys Drop Thread instead of making a GA or try the Ninja Retreat Group.

You can write whatever in a description, but it is never binding. Your rights in every situation is the same:
Either the winner agrees to a deletion (the choice of which a prior description might help influence, but cannot force) OR you take a Not Received (after 7 days).
The only exception is if there is a clear indication that you're withholding your gift on purpose (quite hard to prove). In that case, you risk suspension.