I have had to read this a few times and I fear I still don't get it. Never mind.
If you give away vouchers, then you wont know how much the person winning "invested" in raffle tickets to achieve to win, so in effect paid for the voucher. Could be £20 could be just £1. The treatment is the same and I personally would expect exactly the same treatment whether I paid for it or won it. It's your way of winning over a new client, surely?
If I won a voucher for a treatment that I couldn't use, for example Dermalogica facial as it doesn't agree with me, I would certainly pass it on to someone I knew that would enjoy it. I don't see anything wrong with this, and I wouldn't expect my friend to be awkward or arsey about anything.
I'm not sure if I understand the term goodwill, so I might have this wrong, but isn't the goodwill what YOU have given the charity? That will not be lost no matter who gets the voucher or who comes for the treatment.
I've been selling vouchers for treatments rather than for the amounts. I prefer this and no one has ever said anything to question this. I hope that I haven't misunderstood this, I thought this was normal. I need to look at this too, this thread has made me unsure.
Just one more thing. Do you get a lot of arsey clients? I've not had one yet, but who knows. I had better be prepared.