You didn't say whether the Riffe was a wood gun or a metal tech. If its the breakaway he sells for the wood gun in which a piece of rubber tubing goes through the hole in the butt, I wouldn't use it. Will the shop let you return it?
I shot a tuna with a Riffe Island and that breakaway, and the rubber thing did not pull out of the hole in the butt. It almost tore the gun out of my hands before my slip tip pulled out of the fish, and I am not the only one who has had a similar experience. There is a much simpler and fail-safe system that is usually referred to as the Hawaiian breakaway. I would require you to cut and recrimp your mono, but that is a skill that you are not going to be able to get along without as a spearfisherman, so you might as well get your feet wet now.
To rig the system, cut your mono or cable shooting line so that the loop in the rear end is about 6 inches from the trigger-activated line release. Tie a loop of bungie cord through the loop in the mono and stretch it over the line release. Clip your float line to the loop in the shooting line. When you pull the trigger, everything is released and nothing has to pull out of any holes. Its breathtakingly simple and effective.
The photos show it before and after the trigger is pulled. Unfortunately its shown on two different guns, but don't let that confuse you.