An invitation was given to someone to a Seder on Pesach and they show up with a nice, expensive bottle of Single Malt Scotch Whisky (which is unquestionably barley-based if you don't want to check the link). What Halachicly acceptable alternatives does the host have to react to such a gift at that time?
See this answer
He can accept the gift with intention not to acquire it, and discard it later.
Although it is forbidden to touch chametz on Pesach, for fear that the person will come to eat it (see Orach Chaim 446:3, and Magen Avraham 5; Mishnah Berurah 10), this applies to circumstances where there is a concern for eating the chametz. Under the circumstances, where the chametz is gift-wrapped and sealed, it is possible that this concern will not apply, and there is therefore no problem.
Even if the prohibition will apply, this is not a “formal prohibition,” and under circumstances of kavod ha-brios, where rejecting the gift will certainly cause the person in question shame, one will not have to be concerned for this.
As much as many of us like good single malts, the observance of G-d's mitzvoth are worth more than $400 (more or less).
If the person giving the gift is Jewish then it is Chametz owned by a Jew on Pesach, which makes it forbidden to all Jews to make any use of, forever.
Given you invited them to a Seder, and one does not invite non-Jews to eat at a Seder, I assume that is the case.
You must not embarrass your fellow in public. If it's just the two of you alone and you are able to handle the situation then you might explain it to them. Otherwise you could ask him to put it aside somewhere and dispose of it later. You need to be conscious not to "acquire" it, but you can still dispose of it, because, according to the Halacha, it has no value (see above, no Jew is allowed to make use of it) so you are not "destroying" someone else's property and causing him a loss.
Here is a link about Chametz Sheavar Alav Hapesach (Chametz that was owned during Pesach not being permitted for use afterwards)
Not embarrassing a fellow Jew is so fundamental I will assume I don't need to provide a source.