The Bills Balance is money held in your Debtor's Control, and the Disbursement balance is in your Unbilled Disbursements Control, which is why they are displayed in different office balance summaries.
You can clear these by following the steps below:
From the DPS Home screen select Client Postings.
Select Bills.
Pick up the DPU's in an Internal Bill and Post.
From the DPS Home screen select Client Postings.
Select Allocations.
Tick off the negative and positive bill balances then select Allocate.