I am using DIAPI to create a Sales Quotation.
I would like to update the GrossBuyPr column of the QUT1 table using DIAPI.What property on the Document_Lines should be set to change the value in GrossBuyPr column of QUT1 table?
Well if the field is automatically calculated by the B1 internal logic you better not overwrite it ... as a generic and quite strict rule you cannot write into the B1 DB with a direct SQL statement. All the writes in the DB must be carried out using DI API that encapsulates all the B1 business logic.
If you write directly into the D, your solution looses support and cannot get certified by SAP. Have a look at the solution certification guidelines here on the B1 section of SDN.