Most fish stores will take fish for free as well, you don't have to expect money back. I guess it comes down to what those last two lives are worth you you and your mother. Even the big box stores will accept fish for adoption....
An algae bloom is not uncommon in a newly cycling tank. You need to keep on top of the water changes (at least 25% every other day if every day isn't possible). I would add a bit of salt (1 - 2 heaping tablespoons) if you haven't already and keep the salt dose steady when you do water changes. Also, if you have an airpump, definitely put it to use or allow the water to drop just enough to get some filter splash to help with gas transfer.
If the choice is to continue to keep them, there is no reason to expect they will all succomb. You just have to work very hard to avoid that possibility. A friend with a cycled tank could make all the difference in this situation. With only two goldfish, while it is absolutely not ideal, the tank is more likely to cycle properly and they could remain there for a while (so long as you can keep nitrates below 20 - 40 at the most)