I can't imagine the law regulates who may sign a gift card from a private individual. It's not the kind of thing the law regulates. (A Virginia lawyer may enlighten me to the contrary, but I will be surprised.)
Honestly, I'd be quite surprised if there was a law even close to this issue. Next time, don't be the one to buy the gift, if it might be a problem. Otherwise, I wouldn't make a big deal of who signs, or who doesn't sign. Keep in mind that people who didn't contribute may just be living barely from paycheck to paycheck and might be embarrassed that they can't afford to contribute.
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It seems unlikely that it's illegal. Technically speaking, it's possible that there is a contract issue here, and he may have some sort of contract claim against employees who agreed to contribute and then refused, but unless it was an outrageously expensive gift card, it seems extraordinarily unlikely that it will be worth the cost.
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The best way around this issue is to collect the money first from others and then buy the gift. A contract must be supported by consideration and it is hard to envision what benefit the others would have derived from contributing to a gift.
This is meant as general legal information and is not to be construed as legal advice in an attorney client relationship.