erp tax on update issue?

from function erp_item_update (erp_item.module)

  if (!$node->tax_ex) {
    // Convert prices back to ex tax
    $node->buy_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->buy_price)), 2);
    $node->rrp_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->rrp_price)), 2);
    $node->sell_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->sell_price)), 2);
  }

this code causes the buy_price of an item node to decrease EVERY time an item node is edited and saved

  if (!$node->tax_ex) {
    // Convert prices back to ex tax
    //$node->buy_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->buy_price)), 2);
    $node->rrp_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->rrp_price)), 2);
    $node->sell_price = round(array_pop(module_invoke_all('erp_tax_ex_amount', $node->sell_price)), 2);
  }

commenting out the line about buy_price line stops it but is this right?