Gift Voucher

Not usually, they usually cover them selves in any T&Cs.

A sold voucher is pretty much a sale in the bag to them, so it is like turning down a sale. I had a voucher for a shop and emailed asking about refunding, I was very nice in my email (i.e. not harping on about imaginary rights), and I said if they took it back I would accept a percentage or general cost to cover their handling. e.g. if a shop makes 5% on a typical sale they might not mind handing back this -but accounts wise it might not be easy to do for them. The shop did say I could get a full refund, I forgot and then the recession hit and ~2 years on they wouldn't take it.

The other option is to sell it on adverts or somewhere. Or buy an item and sell it on, buy and item with a good resale value. If you went into the shop you could also chance your arm trying to sell it onto somebody at the counter when they are buying stuff, best clear it with sales men beforehand of course.

Did you try asking them to get in the product you want? I wouldn't even mention the voucher to them if doing this, as they will know you are stuck and might not be as willing to go out of their way.