OK, I found it. In \modules\Product_Options\func.php
PHP Code:
if ($kv == func_get_variantid($v['options'], $productid))
$variants[$kv]['avail'] -= $v['amount'];
replaced with
PHP Code:
if ($kv == func_get_variantid($v['options'], $productid)) {
$variants[$kv]['avail'] -= $v['amount'];
if ($variants[$kv]['avail'] <= 0)
$variants[$kv]['avail'] = 1;
}
It works well for products that current customer put into cart.