Learn AL by writing AL.

codeunit 50100 "Customer Balance"
{
    procedure OverdueFor(CustNo: Code[20]): Decimal
    var
        Customer: Record Customer;
    begin
        if not Customer.Get(CustNo) then
            exit(0);
        Customer.SetFilter("Date Filter", '..%1', Today());
        Customer.CalcFields("Balance Due (LCY)");
        exit(Customer."Balance Due (LCY)");
    end;
}