The Petty cash nominal will be set up as a bank the double entry will effect the Petty cash, the posting will need to effect a different nominal that the payment/receipt is for.
If a new nominal is required then follow the below steps to create one:
Select Nominal tab.
Select Nominal A/C Maintenance.
Click on New Nominal on the right hand side.
Select Next.
Select New Nominal Code.
Enter the number needed for Level 2 or select Auto Generate.
Select Apply, then OK
Select the correct number on Level 1, then Next.
Fill in all the details in the Type/Class/Description.
Select Next and then fill in the Department/Branch/Group as required.
Select Next and Finish.