I think some people are getting confused between a gift card worth actual cash and a voucher code downloaded for free/coupon cut out from the paper which hasn't cost anyone anything.
I was about to say the same thing. Clipping a mass-published random 20% off voucher from the paper (or even having been directly handed one) and claiming that, as it’s ‘your’ voucher, that covers your contribution, rather than throwing it into the pot for everybody, would be in very poor taste.
However, if you've been given an actual cash-equivalent voucher as a gift and/or perk or recognition of work done, it’s just like being given a cash gift or pay bonus. It’s just tax efficient to give an employee a voucher rather than cash, which they would have to pay tax on. I’m guessing that the employer told Karen they wanted to give her a voucher, asked her what voucher she’d like and Karen said “I’m going to XXX restaurant at the weekend, so a voucher for there would be lovely, thanks.”
I also think that Ian probably made sure he got top ‘value’ for his money – deliberately choosing the lobster and cocktails whilst the others had salad and a coke – and then said “Let’s just split the bill”. Then, not happy with being subbed by the split and also by Karen’s kind extra tenner, he decided to brazen it out about the voucher. Apparently, the voucher that Karen had was of no particular intrinsic value yet, curiously, it was also magically able to cover a very large part of the joint restaurant bill.
I also wonder if he's one of those people who confuses kindness (the offer of the extra tenner each) for weakness and goes in for the kill. Or one of these fair-weather communists who sees somebody working extra hours when they’re available to boost the coffers in advance, or buys in bulk to save money – and then reacts by saying “Well, you clearly have plenty to spare, so we should both use yours.”
Supposing he’d gone above and beyond –although he really doesn't sound the sort to do so-- for his employer and he’d been rewarded with a Burton voucher, which he’d used to buy an expensive suit to wear for the meal. Imagine if the others had demanded that he sell the suit the following day and pay a much greater share of the bill in anticipation of what the suit would fetch. That would be completely absurd – but it’s no different at all in principle.