+  my $taxdate = $form->{tax_point} ||$form->{deliverydate} || $form->{invdate};
+
+  # Sanity checks for invoices for advance payment and final invoices
+  my $advance_payment_clearing_chart;
+  if (any { $_ eq $form->{type} } qw(invoice_for_advance_payment final_invoice)) {
+    $advance_payment_clearing_chart = SL::DB::Chart->new(id => $::instance_conf->get_advance_payment_clearing_chart_id)->load;
+    die "No Clearing Chart for Advance Payment" unless ref $advance_payment_clearing_chart eq 'SL::DB::Chart';
+
+    my @current_taxaccounts = (split(/ /, $form->{taxaccounts}));
+    die 'Wrong call: Cannot post invoice for advance payment or final invoice with more than one tax' if (scalar @current_taxaccounts > 1);
+
+    my @trans_ids = keys %{ $form->{amount} };
+    if (scalar @trans_ids > 1) {
+      require Data::Dumper;
+      die "Invalid state for advance payment more than one trans_id " . Dumper($form->{amount});
+    }
+  }
+
+  my $iap_amounts;
+  if ($form->{type} eq 'final_invoice') {
+    my $id_for_iap = $form->{convert_from_oe_ids} || $form->{convert_from_ar_ids} || $form->{id};
+    my $from_order = !!$form->{convert_from_oe_ids};
+    my $invoices_for_advance_payment = $self->_get_invoices_for_advance_payment($id_for_iap, $from_order);
+    if (scalar @$invoices_for_advance_payment > 0) {
+      # reverse booking for invoices for advance payment
+      foreach my $invoice_for_advance_payment (@$invoices_for_advance_payment) {
+        # delete ?
+        # --> is implemented below (bookings are marked in memo field)
+        #
+        # TODO: helper table acc_trans_advance_payment
+        # trans_id for final invoice connects to acc_trans_id here
+        # my $booking = SL::DB::AccTrans->new( ...)
+        # --> helper table not nessessary because of mark in memo field
+        #
+        # TODO: If final_invoice change (delete storno) delete all connectin acc_trans entries, if
+        # period is not closed
+        # --> no problem because gldate of reverse booking is date of final invoice
+        #     if deletion of final invoice is allowed, reverting bookings in invoices
+        #     for advance payment are allowed, too.
+        # $booking->id, $self->id in helper table
+        if (!$already_booked) {
+          # move all netamount to correct transfer chart (19% or 7%)
+          my %inv_calc = $invoice_for_advance_payment->calculate_prices_and_taxes();
+          my @trans_ids = keys %{ $inv_calc{amounts} };
+          die "Invalid state for advance payment invoice,more than one trans_id" if (scalar @trans_ids > 1);
+          my $entry = delete $inv_calc{amounts}{$trans_ids[0]};
+          my $tax;
+          if ($entry->{tax_id}) {
+            $tax = SL::DB::Manager::Tax->find_by(id => $entry->{tax_id}); # || die "Can't find tax with id " . $entry->{tax_id};
+          }
+          # no tax, no prob
+          if ($tax and $tax->rate != 0) {
+            my $transfer_chart = $tax->taxkey == 2 ? SL::DB::Chart->new(id => $::instance_conf->get_advance_payment_taxable_7_id)->load
+                              :  $tax->taxkey == 3 ? SL::DB::Chart->new(id => $::instance_conf->get_advance_payment_taxable_19_id)->load
+                              :  undef;
+            die "No Transfer Chart for Advance Payment" unless ref $transfer_chart eq 'SL::DB::Chart';
+            $form->{amount}->{$invoice_for_advance_payment->id}->{$transfer_chart->accno} = -1 * $invoice_for_advance_payment->netamount;
+            $form->{memo}  ->{$invoice_for_advance_payment->id}->{$transfer_chart->accno} = 'reverse booking by final invoice';
+            # AR
+            $form->{amount}->{$invoice_for_advance_payment->id}->{$form->{AR}} = $invoice_for_advance_payment->netamount;
+            $form->{memo}  ->{$invoice_for_advance_payment->id}->{$form->{AR}} = 'reverse booking by final invoice';
+          }
+        }
+
+        # VAT for invoices for advance payment is booked on payment of these. So do not book this VAT for final invoice.
+        # And book the amount of the invoices for advance payment with taxkey 0 (see below).
+        # Collect amounts and VAT of invoices for advance payment.
+
+        # Set sellprices to fxsellprices for items, because
+        # the PriceTaxCalculator sets fxsellprice from sellprice before calculating.
+        $_->sellprice($_->fxsellprice) for @{$invoice_for_advance_payment->items};
+        my %pat = $invoice_for_advance_payment->calculate_prices_and_taxes;
+
+        foreach my $tax_chart_id (keys %{ $pat{taxes_by_chart_id} }) {
+          my $tax_accno = SL::DB::Chart->load_cached($tax_chart_id)->accno;
+          $form->{amount}{ $form->{id} }{$tax_accno}  -= $pat{taxes_by_chart_id}->{$tax_chart_id};
+          $form->{amount}{ $form->{id} }{$form->{AR}} += $pat{taxes_by_chart_id}->{$tax_chart_id};
+        }
+
+        foreach my $amount_chart_id (keys %{ $pat{amounts} }) {
+          my $amount_accno = SL::DB::Chart->load_cached($amount_chart_id)->accno;
+          $iap_amounts->{$amount_accno}                 += $pat{amounts}->{$amount_chart_id}->{amount};
+          $form->{amount}{ $form->{id} }{$amount_accno} -= $pat{amounts}->{$amount_chart_id}->{amount};
+        }
+      }
+    }
+  }
+
+  if ($form->{type} eq 'invoice_for_advance_payment') {
+    # get gross and move to clearing chart - delete everything else
+    # 1. gross
+    my $gross = $form->{amount}{ $form->{id} }{$form->{AR}};
+    # 2. destroy
+    undef $form->{amount}{ $form->{id} };
+    # 3. rebuild
+    $form->{amount}{ $form->{id} }{$form->{AR}}            = $gross;
+    $form->{amount}{ $form->{id} }{$advance_payment_clearing_chart->accno} = $gross * -1;
+    # 4. no cogs, hopefully not commonly used at all
+    undef $form->{amount_cogs};
+  }