Take the remaining Damsel back to your LFS
if they'll have it back. If they'll hold it for you, great. If not, it's better than killing it.
All you'll be doing if you keep it in there is watch it die.
Damsels are hardy, and many survive the cycling process, but a lot of them won't get back to full health, leaving them more prone to problems in the future and therefore affecting the stability of your tank and future livestock. They may live during the cycle, but it must be like living in a pig sty breathing in smog and sewer waste. It's just not fair...
Use another method of cycling (raw shrimp etc.) which is less hassle, better, and humane!