Floating point numbers are simply not suitable for storing exact but arbitrary numeric values; every floating point number is just an approximation of a numeric value. Sure, some numbers can be represented exactly as a floating point value but many cannot and then you will have an error that can vary a lot, so it's better to always treat them as an approximation. E.g. even a simple value as 0.2 cannot be exactly represented as a double value. Calculating 0.2 + 0.1 in C# (assuming both are double values) results in 0.30000000000000004, whereas 0.3 would have been correct.
Money can actually easily be represented as integers, just store it as cent values. A 64 bit integer value is more than suitable for this task. During calculations you can convert it to higher precision values first (like decimals), perform the operation, and finally convert it back to cent at the end, using appropriate rounding but for many tasks you don't have to (addition, subtraction, and multiplication will be fine with integer values).
The problem with determining the error is that errors sum up. If you calculate interests, the error will be too small to be significant but if you calculate compound interests of a huge amount of money and then use the result in other calculations, the error can become significant. So if you just look what the error of a single operation is, then this may not be useful as this operation may be used as one operation in a set of operations to calculate a single result in the end.
Also errors can sum up over the years. If you store an account balance as a double and always round the value to two digits after the period for displaying but internally keep the full precession, you may get correct results for years and after 30 years all of a sudden you will be off by one cent which does not only depend on the balance of the account but also on the number of transactions over the years. Sure, most people will think "One cent, is nowhere near significant". Well, if you are a finance institute and you have 100 million accounts and each is off by just one cent, your total balance is off by 1 million dollar and I would call that significant.
I'm not sure of what exactly your code is calculating and how often a day it is doing so without interim rounding, so I cannot really provide a better answer here. E.g. if the money is really just transferred (subtracted from account A and added to account B), using double will not easily cause significant issues. You usually need more complex operations that include multiplications, exponentiations/square roots to quickly accumulate significant errors.