Suggestions for speeding up working on Pre-payment Sales Orders

We are using Navision 5.0 SP1.

Right now when we have a Sales Order with a prepayment no work can be done on getting the parts built, ready to ship, etc. until the Prepayment has been met.

Any suggestions on how we can get the part ready to ship but not allow the system to ship part until the prepayment is met?? While were waiting for the prepayment were getting further behind on getting part ready to ship and once the prepayment is met it’s a mad rush trying to get part out on time.

Thanks in advance,