Take a look at Is it safe to change order status before save?

Assuming that your payment is set to authorize and capture

Once an invoice is generated then the status should be processing because you should already receive payment (take a look at cash/check payment method once you invoice balance due become zero)

But if you still want to make it “pending payement”

in system.xml you need to either create a new source_model or find one that has pending payment status, the set it as default in system config your payment method

<order_status translate="label">
  <label>New order status</label>
  <frontend_type>select</frontend_type>
  <source_model>adminhtml/system_config_source_order_status_processing</source_model>
  <sort_order>4</sort_order>
  <show_in_default>1</show_in_default>
  <show_in_website>1</show_in_website>
  <show_in_store>0</show_in_store>

</order_status>

Take a look at Is it safe to change order status before save?

Assuming that your payment is set to authorize and capture

Once an invoice is generated then the status should be processing because you should already receive payment (take a look at cash/check payment method once you invoice balance due become zero)

But if you still want to make it “pending payement”

in system.xml you need to either create a new source_model or find one that has pending payment status, the set it as default in system config your payment method

<order_status translate="label">
  <label>New order status</label>
  <frontend_type>select</frontend_type>
  <source_model>adminhtml/system_config_source_order_status_processing</source_model>
  <sort_order>4</sort_order>
  <show_in_default>1</show_in_default>
  <show_in_website>1</show_in_website>
  <show_in_store>0</show_in_store>

</order_status>