Your ledger is correct. The negative office balance is the VAT element on the disbursement, which is in the bill, but you have zero rated the VAT. Once you receive full settlement of your bill and your anticipated disbursements get settled you will find that your office balance returns to nil. Please see an example of what will happen below:
I post an anticipated disbursement for £500.00 plus £100.00 VAT.
I then pick the disbursement up in a bill but I zero rate the VAT, the bill total is £500.00, and this creates an negative £100.00 office balance.
My client then pays the bill in full for £500.00, which gives me a negative £600.00 office balance.
I then pay my disbursement out in full for £500.00 plus £100.00 VAT, which results in my office balance returning to nil.