account-reconcile/invoicing_voucher_killer/invoice_view.xml

<openerp>
  <data>
    <record id="invoice_voucher_group" model="ir.ui.view">
      <field name="name">Hide voucher in invoice</field>
      <field name="model">account.invoice</field>
      <field name="inherit_id" ref="account_voucher.view_invoice_customer" />
      <field name="type">form</field>
      <field name="arch" type="xml">
        <xpath expr="//button[@name='invoice_pay_customer'][last()]"
               position="replace">
          <button name="invoice_pay_customer"
                  type="object"
                  string="Register Payment"
                  attrs="{'invisible': ['|', ('state','!=','open'), ('sent','=',True)]}"
                  groups="invoicing_voucher_killer.invoice_voucher_user"/>
        </xpath>
        <xpath expr="//button[@name='invoice_pay_customer'][1]"
               position="replace">
          <button name="invoice_pay_customer"
                  type="object"
                  string="Register Payment"
                  attrs="{'invisible': ['|', ('state','!=','open'), ('sent','=',False)]}"
                  class="oe_highlight"
                  groups="invoicing_voucher_killer.invoice_voucher_user"/>
        </xpath>
      </field>
    </record>

    <record id="invoice_voucher_group_supp_inv" model="ir.ui.view">
      <field name="name">Hide voucher in supplier invoice</field>
      <field name="model">account.invoice</field>
      <field name="inherit_id" ref="account_voucher.view_invoice_supplier" />
      <field name="type">form</field>
      <field name="arch" type="xml">
        <xpath expr="//button[@name='invoice_pay_customer'][last()]"
               position="replace">
          <button name="invoice_pay_customer"
                  type="object"
                  string="Pay"
                  states="open"
                  class="oe_highlight"
                  groups="invoicing_voucher_killer.invoice_voucher_user"/>
        </xpath>
      </field>
    </record>

  </data>
</openerp>