Delay tast cost for 1 month

[Unitrip]

Here is an example of what I need to do:
I have a task that will take 5 Weeks
All my cost are task based not resource based.
This task will cost $1000 and it is prorated.
I don't want the cost to come out of my budget until I recieve an invoice
which I recieve on the 5th of every month.
The cost have to be prorated because I get invoiced for % Complete at
billing date

I am trying to use this for a cash flow report and can't seem to get it right.