Personally, i'd release it, or send it to me :P. I've been looking for an octo species for a while now.... people just charge an arm and a leg for them. Any chance you could get a pic?
If you were to keep one- you'd need a species only tank, an ample supply of foods with a few types of live crabs, snails, and hermits, and depending on the species, require a 7 gallon tank and up (55 for common ones). Unless you have a good pic, it's pretty hard to identify it. You may be getting dwarf species, or a larger type.
Check out the octopus site/forum if you need any help (not posting a link because I know what will happen...).