Many clients I work with have a tough time selecting appropriate candidates, which often costs them big time. At the end of the day, you can do all the checks and balances in the world, but you'll never truly know how successful someone will be in your role unless you make a decision and give them a chance.
If you have reference checked, conducted a thorough interview process, and been honest and open with the candidate throughout the entire process, then there's really little more you can do to control the situation. Sometimes, things just don't work out when you appoint a staff member, but it's important no to let that fear hold up your decision.
If you're waiting on your manager's sign-off, then you really need to beat down their door to have an outcome decided on. I cannot stress this enough - elite candidates will not wait for your inability to make a decision or for you to not prioritise your recruitment process. The best candidates have multiple opportunities available to them, and they're assessing your process.
The excuse of "I'm busy" doesn't fly here either. If you're that inundated with work, then it makes sense to take one step back, for two steps forwards. In this case, it means hiring help to assist you with your workload.
Be thorough, but speedy with your recruitment process.