There are proper checks and balances in place and they work if they are followed. The brother collecting has to have another brother there with him when he collects and counts and records it on the receipt slip. They both sign (initial) and turn it into to an elder (usually the secretary in the congregations I attended). A different secondary brother is used each time. Believe me the accounts servant doesn't want to given anyone room to accuse him of dishonesty. Those slips are used later when the 3 month audit is done or delegated by the PO. The only time I have seen money go awry is when the elders do not do a proper audit, usually due to incompetence. One note about contributions, the report given each month is for the previous month. So if you donated a sum in March, the report read in April is for February. BTW if you donate $250 or more to any charity (non-profit organization) in the US, the IRS requires a receipt from the organization to prove you donated.
Also, when you donate to the congregation, it is hard to designate how it is spent unless you set up some legal arrangement, signed and witnessed. BTW every November or December the WTS has an article as to how you can give conditional donations legally. But I know of cases where they have tried to renege on that...but the individual prevailed due to their persistance.
Blondie (accounts servant's wife)