Kot, Jack, the problem our OP is getting at is that, by turning in the foci, they're putting the mage behind bars, functionally forever.
Assuming, Sonarean, that LS isn't using the claim as a ploy to get themselves some free Foci, as Kot originally suggested. The evidence is turned over to LS, and goes missing a few days later during a "routine fire drill" of the evidence building.
Of course, if you want LS to be more "White Hat" good guys (it sounds like you're group has a strong good v. evil, White Hat streak), Mr. Mage could have been keeping a bound spirit (or an Ally Spirit or Free Spirit bound via a Spirit Pact, if his regular spirits are unimpressive) on standby for just such a contingency.
It is now acting out the orders of "acquire all of my property which was stolen from me and eliminate the rest of the evidence". As such, it's laying in wait (spirits have Sapience and Logic = Force, anything over Force 3 is going to make a pretty decent plan, not barge in head first), following his master's astral signature to the foci, and ambushing the LS fools who take custody of the evidence.
Alternatively, the spirit is instructed with a simple "set me free of the prison", and breaks the bum out. Player's have given their all, so they don't feel terribly cheated, and the mage lives on to fight another day.
As long as you don't make the players feel like they're impotent to stop Mr. Mage from escaping, they'll generally accept story complications that unravel their efforts. You can reinforce their own competence by having LS turn to them to help re-capture the lost foci / mage. On the other hand, they might blame the team for trying to "double dip", get the reward for turning in the evidence, then using magic to steal it back.
Plenty of options to consider. But, the key is, remember that your baddy is a powerful mage, presumably, he's smart enough to have considered at least one, if not several, viable contingency plans in the event of his capture. Perhaps he wanted to be caught, to gain plausible deniability for the next phase of his diabolical plot. After all, he couldn't possibly have committed that crime from prison. Or, he's there to help with a break out of an even bigger bad guy (or very loyal underling).