There is an interesting real estate rule that says, when your home is listed on MLS, you must allow access to your home within 24 hours of a request of a showing.
It is a strange one I know but one to be aware of. This means if an agent requests a viewing, on an active listing that you will need to offer up a time that works for you within 24 hours.
Most agents will be totally reasonable about this as long as you are responsive!
There are a few ways to handle this situation:
If you are not home - you can hide a key, use a lock box or provide access through your security system.
Suggest another time right away that works for you showing you are responsive.
If you are conditionally sold, let the agent know as they might opt to not see it.
If you are conditionally sold and do not want to allow any more showings, you can make your listing "pending" on MLS. If you decide to do this, agents will see this information and if the deal falls through, it may be a bit of a red flag.
Let us know if you have any specific questions about how to deal with this for your listing.