A small business tends to minimize the number of staff. This leads to combining tasks and put them under one person responsibilities.
How best should we do it to avoid fraud.
This significant issue is missed in most startups: how to build a framework that supports the business you are in. These issues are what can divert many founders from the vision, mission, and ambition, into administrative roles. I have seen as many businesses fail for this as I have seen for product failure. Without bias, fraud or incompetence, internal controls should be developed for both. Fraud is the most aggressive, yet unintentional damage through incompetence is more prevalent and preventable. Early in the process you need an operations person that understands basic accounting, operations, and HR who is fiercely loyal to you. You must be involved to set policy on spending and revenue recognition, partnership and distribution agreements, and legal issues. You need to understand basic HR and what documents you need to protect you and the employee, and what policies must be in place, or anticipated. You need an outside accountant to review the books and records a minimum of annually, but if you are not convinced your internal person is highly skilled, then quarterly. A lot can go wrong in a few months with a small business. This is a lot to learn, but can be developed over time with outside resources. However, your fraud question leads me to think you are aware that what can go wrong will go wrong, a very good sense to have early on; not paranoia, just awareness.
If you are worried about the possibility of fraud by giving too much responsibility to your people, I think you may have deeper problems than you think.
Either you have hiring issues or trust issues. The only way to grow and succeed is by hiring good people, giving them the tools and information they need to do their job, and trusting them to do their job.
Having said that, people make mistakes and forget things. So, I try to build checks and balances and automation into my systems so we don't leave it up to human memory and minimize the chance of human error.
Staff shifting where by one staff does not stay in one position for too long