I Think My Employer Is Trying To Frame Me For Stealing

I woke up to an email from my employer today saying that the deposit from the day that I closed dropped from $200 to $2.74. I have been closing at this shop for the past 5 years and I have never come across something like this before. I would have definitely noticed if the deposit for that day was significantly lower than what it normally is and would have reported.
To add a little bit more context, I feel like the relationship between my employer and I has soured ever since I voiced my concerns over how some things are managed at the shop and they're trying to find any probably cause of firing me. How can I protect myself in this situation and how do I respond to their email inquiring about the discrepancy in the deposit?