Question on Setup and Billing/Revenue for Grant with Changing Pre-Paid Amounts and Events

I am looking for some advice on how best to handle a pre-paid grant/contract where the pre-paid amounts and events are not known at the time of grant setup. Thus, the grant/contract have been set up as a typical cost-reimbursable grant/contract.

My understanding from the client is that the grant sponsor contacts them throughout the grant and asks them to submit a billing for costs that have not yet occurred if there is a chance the grant sponsor would not able to pay promptly if they waited until the costs had occurred and the client submitted the billing after the fact (for example the possibility of a government shutdown). However, most of the costs for the grant are still billed to the sponsor after occurring.