Payments and allocations to supplier trust accounts

Our company has retained a lawyer to commence legal action in relation to some issues with damages to hired out equipment.  Prior to commencing work and throughout proceedings, we were required to pay amounts to their trust account.  These payments were entered as Spend Money transactions (including GST) as we had no invoices to allocate towards.

As work is progressing, the law firm is sending though invoices and requesting approval to withdraw funds from our trust balance.  How do I deal with this within MYOB? I want to ensure that GST is accounted for accurately. Initial trust funds were deposited last financial year so I'm not sure if this will complicate matters further.