Dear,
1.With Regular Invoice ,you can not done Down payment.
2.Milestone Billing can not fit this .
3. Create a payment condition like 20% - 0 days rest in 30 days and issue an advanced payment invoice and later an invoice for the rest
Choose the third option and tell your FI guy about this requirement .He will create Payment as based on your requirement or else go by Periodic Billing Process .It would be also fit for this .
Thanks,
Naren