Secrecy is SSPX's modus operandi. Not just about money. About everything.
I have heard that they will demand legal possession of the chapels to give people the sacraments. They will only say mass on someone else's property if it is a temporary solution, like on a mission, for instance.
Frankly, I don't give them much money, and if they misuse it, it's their problem, not mine. I give in good faith. If money was the only problem, I would be very happy. There's just too much trouble in my life for me to care about how the SSPX handles the money. No disrespect to you intended, of course.
The case does sound fishy, but if I were to bet money on it, I would bet on incompetence.