Your issue is that you are talking about BOTH Sales Orders and Invoices.  A Sales Order does not reserve inventory the way an Invoice does when you enter a Lot or Serial number. Well unless you are committing Lots and Serial Numbers during Sales Order entry. 

The common way is to rebuild the sort files. First post all invoices, make sure the invoice data entry files are empty. Then rebuild the inventory and sales order sort files. When you do inventory it will ask you if you want to clear committed, answer yes. That should fix it.

It's telling me in IM that  the items are committed on sales orders. There's only one sales order with the item on it, and the quantity on that sales order is 3.5, but I'm being told that 7 are committed.I'm using Sage 2018 with data levels at 6.0, if that makes a difference.

We have been entering everything into sales order, and converting them to invoices just before we update. We keep the invoice data entry files empty as much as possible for the sake of our employee who works remotely and is 6 hours ahead of the rest of us. I take it that we can rebuild files with sales and purchase orders entered without creating issues?

It is best to rebuild the files when there are no unposted invoices.  Rebuild the sorts for inventory and sales order and clear committed.

That won't affect open Sales Orders? We are unlikely to ever have all Sales Orders converted to invoices and posted.

Rebuild sort files on SO recalculates the "On SO" / "On BO" values for each warehouse, considering open orders.  Rebuild sort files on IM does a similar correction on cost tier data (lot / serial quantities).