Custom Dropdown/Lookup Field

I have created an asset management site using a Main Asset Form, an Assign Asset Form, and an Asset Return Form. Essentially the Main Asset Form is used to create all assets and then the other two forms are used to sign assets out and then log their return. I have a Lookup Dropdown field ("*Asset") in the Assign Asset Form that pulls a list of all the assets that are created using the Main Asset Form.

I would like to set up the Lookup Dropdown field ("*Asset") in the Assign Asset Form to automatically remove all assets that are signed out (previously selected in a form submission) but then return them to the list once they are returned. Therefore only displaying assets that are available for sign out.

Here is the formula I can see (my simple way of explaining what I am looking to do).

Main Asset Form - Asset Sign Out Form + Asset Return Form = Asset List Displayed in the Lookup Dropdown field of the Asset Sign Out Form.

Can anyone help with this or give guidance on this?

Thank You

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