Follow the steps to setup a way to take petty cash out of the till and register it with your reporting.
Add a product in your admin console or within the POS settings called "Petty Cash" and set the price to £0.00 and the Tax to 0%. Set this product to active.
Add a new category called "Misc" (or "Z Misc" if you would like it to appear as the last tab on your POS).
Add the "Petty Cash" product to the "Misc" category.
Refresh the POS products / categories (settings > utilities > "refresh products and categories".
See https://getshopwave.zendesk.com/hc/en-us/articles/201922731-Add-a-product for more about how to add products.
When you are needing to register petty cash being taken out of the till then follow these steps:
When using any of the reporting tools in the admin you can search for your product "Petty Cash" and then view the performance for that over the time period you select.