Restrict Updating Selected Fields of Incomplete Requisitions

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We receive orders from multiple clinical systems, which are imported as Incomplete Non-Catalog requisitions. Users review these requisitions in SSP, update fields such as Quantity, Location, Charge Account, and add approvers and then submit them for approval.

However, per our business requirements, users should not be allowed to modify certain fields like Item Description, Category, UOM, Supplier, Supplier Site, Supplier Item Number, Line Price, and Currency on these requisition lines.

Could you please suggest if there is a suitable mechanism to enforce this restriction?
Additionally, in future releases, would it be possible to make these fields read-only using VB Personalization, either based on Interface Source, Line DFF, or for all requisitions (we can disallow users from initiating the requisition from SSP) ? Thank you