As part of the Purchase Orders / Goods In process, you can view all the Customer Orders that will be fulfilled from this stock and print an A4 receipt to attach to those items when putting them to one side.
To do this, open the Purchase Order in the Back Office and click on the View Customer Orders button near the bottom of the screen.

This will open a new window showing all applicable Customer Orders:

You can then click on Actions on the left to open the customer card, open the customer's order in POS, send them an SMS or email, change the status of their order, or click Open Full-Size Order Receipt which will print a copy of the A4 receipt that you can attach to the item.
This screen will also appear when you commit the stock at Goods In, after it prompts you to print out the barcode labels for the stock you've received.