Hi
In your calculation you are already deducting retain amount from net value then where you want it to post. What do you mean by customer ledger. If you have assigned acount key with this condition then it must be posting to some GL and for customer it is already deducted from net value.
Why don't you use SAP standard milestone functionality for this. It looks like that it can be controlled with milestone billing. HAve you explored that before suggesting this solution to client?
Thank$