In California gift cards are the same as cash - they do not lose value over time. If you pay the full value of the card, it is good for that amount. If someone at Joanne's said it cannot be used the same as cash I would ask them to cash out the card and use the cash to buy the machine. The Joann's in my area does not have a separate sewing machine section, so if you buy one from their display you take it to the same check out stand. If their sewing machine area is owned by someone else, then that may be different.
Our JC Penney's store has a separate department for Sephora and when you purchase from them you pay Sephora, not JCP - so if they rent space from Joann's they would not be held to the same standard.