Hi Aboul,
if A/P invoice payment without linking to A/P down payment, A/P down payment will invalid. So, you should cancel A/P invoice, after that, you make new A/P invoice (100% not 70 %) which linking to A/P down payment (30%). but, you make sure A/P down payment is received (A/P Down payment -> Outgoing payments)
Try to follow process which i mentioned above.
Hope this helps.
Regard,
Hang Bui