How to check multiple checkboxes on a page based on conditions

Hello everyone,

I'm having trouble with a feature I'm trying to implement:

I have this "Data Page" that shows Customers and their respective invoices below them:



So I'm trying to click on the checkbox from the line where "Type = Customer" (Marked red)
and the lines where "Type = GL Account" should get checked (marked Blue).

I have tried this so far on the page trigger:

Approved - OnValidate()

rec_data.SETRANGE("Account Reference",Rec.Account);
IF rec_data.FINDSET THEN BEGIN
REPEAT
//MESSAGE('%1',rec_data.Account);
Approved := TRUE;
UNTIL rec_data.NEXT = 0;
END;
/.../

rec_data is a Record variable with the information.
"Account Reference" is a field which contains the Customer No. so that each Invoice knows who it belongs to.
The line "MESSAGE('%1',rec_data.Account);" correctly displays all of the invoice's numbers that are under the selected Customer but the "Approved := TRUE" does not work.

I'm out of ideas, also I don´t know if this is the correct way.

Please help me, thanks in advance.
Cristiano Ferro.