How to update lookup data from a picklist depending on its status?

Hello everyone,

I'm new to Zoho CRM and working on customizing it to fit my company's needs.

Currently, I have a module called "Units" (similar to products) that represents the units available in a project (construction project).

My deals are split into two stages: the sales process stage and the booking stage.

In the sales process stage, it's not necessary to assign a deal to a specific unit. However, in the booking stage, the deal must be linked to a unit number.

My question is: how can I update the lookup field (unit number) to only show unassigned units, preventing the same unit from being assigned to multiple deals?

I'm considering adding an "Availability Status" field in the Units module (e.g., Available - Sold Out), and based on this status, the unit would not appear in the lookup field. However, I'm not sure how to implement this in the layout.

I hope I've explained everything clearly. Thank you in advance for your help!

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