How to record Down payments (client up front payments)

How to record Down payments (client up front payments)

how can i record and track down payments from my costumers?

case:
travel agency

i record and sent an invoice on 1. january 2012 with 100 usd to my costumer
he is requested to pay 10% of this invoice till 15. january
and the balance on 15. april 2012

i dont want to have to whole 100 usd in my january accountings as income - but the 10% the costumer has paid

i want to have to balance of 90 usd accounted as receivables in april.

how to do this in zoho books?