It looks like a valid move from you, but you probably should be prepared for some questions. After border officer scan your ID, they definitely will get all information about you, and with high probability they'll see your visa. You might be asked about it, if it still present, and do you have it with you (as if you've lost it, they need to cancel it as quick as possible). Next question will be, why aren't you using your visa. If it allows multiple re-entries, then you're fine, just losing some time at the border.
However, it's really easy for you just to use your visa, I think.
Update: as from comments, it looks like your visa is single-entry, so, it's probably not the best idea to check the system, as you could look very suspicious: FAN id is working only for 10 days after FIFA ConfCup, and your visa is till September, so, technically speaking, you're breaking the law, entering the country second time, as you clearly not going to return from Russia after FIFA Id ends.
Even if you manage it out, you still could face some troubles in future with getting other visas, as the embassy stuff will see that you've entered Russia two times with a single-entry visa. So it's really slippery road, which can bring you more troubles than fun.