Moving records like this from one table to another is not a good idea. There are much safer/simpler options that will do what you need.
1) I don't recommend a single table. You may even end up with more than two in the final result, but you'll need at least two, but not as you have imagined.
2) well moving entire records from one table to another is usually not the best solution and there are better options for what you want.
You have one record in Inventory for each item or group of identical items. Each time some event increases or decreases inventory, you record that event in a new record in Transactions. Summary totals that add up all transactions that increased inventory for a given item and that subtract out all transactions that decreased the count compute the current "on hand" value.
The Inventory starter solution that comes with FileMaker 12 illustrates a simple version of this approach.
This thread, which takes this forward into more complex applications of the same method, may also be of interest to you: Managing Inventory using a Transactions Ledger