No problem, you can always add expenses that are paid in cash, or any other mean of payment.
1/ Create the expense & attach the proof.
Add the expense like any other expense, using the '+' button on the 'Expenses' screen or from the 'Bank Account' screen.
Attach the receipt if you can and make sure the fields are correct. It's important for the computation of your taxes.
2/ Select 'Match remainder with cash' in the "Link Payment' menu
In the lower part of the screen, click on 'Link payment' and select the cash option in the list.
Off you go!
Was it not clear? No worries, we are available in the chat & email.