Purchase Order POR1 table lock Field

[hemish]

Hi,

I'm stuck and needed some help.

On my purchase order in SAP Business One we have the field "ShipDate" i have created a UDF field called U_InDelDate, and created a FMS so the Ship date = U_InDelDate. This works not a problem. But the issue i'm facing is i only want the U_InDelDate to be changed once, after the first time U_InDelDate has data i don't want this field to be changed again. Field ShipDate they can change as many times as they want.

I've created a store procedure at the time of update i get the error message saying you cannot update again, but problem is if user has changed the date on 6 lines they need to revert back and might not be able to remember the dates. 

Is there a way the field can only be updated once and if they try to put the record again it won't accept it? The data base we are using is Hana.

Thanks