I have heard that some stores might give store credit. I suspect it will be the mom and pop stores. I asked at Petco a year or two ago and they said they would not give me money nor store credit but would accept the fish if they were in good condition. When I was purposely breeding fish I managed to find plenty of neighbors who would take them for free.
If you don't want more babies you will need to separate the males and the females as soon as you can tell them apart to make sure they don't mate. I would check at some local mom and pop stores now. Some of them may take the fish as soon as they are a quarter inch or half inch long. I have seen some fish that size for sale at some of the stores I shop at.