Problem in updating Sales Invoice Header table through code

a credit memo moves to the "Sales Cr.Memo Header" so are you saying you are applying the cm to a posted sales invoice and you want the applied invoice to be updated?

what's your code?

I'll assume you have the code on the OnAfterValidate of the "Applies-To Doc. No."
I've added the message field so you can see when the value exists.
If you try it again you'll see if you use the lookup to get the Invoice number- your code doesn't work.
But if you copy the invoice number and paste it into the "Applies-To Doc. No." it will!
the lookup is not triggering the OnAfterValidate.

OnAfterValidate()
IF SalesInvHeader.GET("Applies-to Doc. No.") THEN BEGIN
SalesInvHeader."Reason Code":= "Reason Code";
MESSAGE('%1',"Applies-to Doc. No.");
SalesInvHeader.MODIFY;
END ELSE BEGIN
MESSAGE('Invoice No. not Found');
END;

But with a regular customer license you should then get an error saying you're not allowed to modify the Sales Invoice Header Table. Then (for me) I would then have to edit CU391 to allow me to change that field.

Are you a partner or a customer?

SalesInvoiceHeaderEdit(SalesInvoiceHeader : Record "Sales Invoice Header")SalesInvoiceHeader2 := SalesInvoiceHeader;
SalesInvoiceHeader2.LOCKTABLE;
SalesInvoiceHeader2.FIND;
SalesInvoiceHeader2."Reason Code" := SalesInvoiceHeader."Reason Code";
SalesInvoiceHeader2.MODIFY;
SalesInvoiceHeader := SalesInvoiceHeader2;

I know this makes the field editable but not sure if updating it thru code from another form will work. Testing is needed & your partner should help.
I'm guessing it should still crash on -> SalesInvHeader.MODIFY;

Ever thought about using the Credit & order Comments?