How can I buy single items from a merchant in NWN?

I'm playing Neverwinter Nights (iOS version, if it matters), the Shadow of Undrentide campaign, and I'd like to buy one ruby from a merchant. However, her inventory is showing two rubies and it seems I can only buy them together. Neither the radial menu nor the inspection option offers the option to split or buy only a single item. Rubies are pricy so I'd rather avoid buying them both (I probably shouldn't have sold them in the first place, but hindsight is 20/20 ...)

You can't, unfortunately. Items in a merchant's inventory are provided "as-is", including stack count. Normally items sold by the vendor will be singles, except for ammunition, but items you sell to them stay stacked as you've seen. The only way to end up with one ruby would be to buy both, split them, and sell one back (at the normal vendor rate).

As far as I know, this is true for all platforms, versions, and expansions.