Make your systems strong.
Have a clear process: where does the money come from, where does it go?
Account for it transparently: how are movements of money planned, approved, recorded, tracked and evidenced?
Have checks and controls: Who sets things up? Who approves payments? Who checks? Who checks the checker?
Minimise temptation and opportunity.
But at the end of the day, if you can’t trust someone – don’t work with them.