Hi,
There is no option to separate an order payment in two.
What you can do, however, is to create a auto loaded coupon of 70% via the menu System->Discounts. That will give a 70% discount on the checkout automatically so that the user will only pay 30% upfront. You will need to change the wording as it states 'coupon' by default, and set "amount left to be paid" via translations override:
www.hikashop.com/fr/download/languages.html#modify
and remove the "coupon" view from your checkout workflow option in the configuration.
However, that means that you won't be able to issue coupon codes for discounts to your customers.