The RMS program workflow is to process back orders with no deposit or full deposit*. This is set by the parameter under Manager, File, Configuration, Options tab, POS options, 'Require full deposit on back order'. If enabled, the customer is required to make full payment for the order. If disabled, no deposit is required.
If you would prefer to take partial deposits on back ordered items, use Work Order form instead (Ctrl-F2). Press Shift-F9 to view the work order details and override the deposit amount if necessary. Otherwise the deposit amount will be calculated based on the setting in Manager, File, Configuration, Orders tab.
*Latest version of RMS allows variable deposit % on backorders, but workorder is still the recomemnded form to use, especially when there are multiple items on the order.