If you have a backorder quantity in excess of a quantity on a single purchase order, you have to split the sales order line quantities to be the same or less than the quantity on the purchase order. ie: if you have 54 on back order, you can only assign 25 to the first purchase order of 25, then split the line to a second line for 25 for the next purchase order of 25, then have a third sales order line for 4 for the next purchase order. It looks ridiculous on the sales order. The open purchase orders are displayed with the available quantities so you would know how many can be assigned to each open purchase order.
Unfortunately, this is a GP limitation that we can't really workaround due to the fulfillment of the sales line being handled by GP and not by SalesPad. They allow you to tie a PO to multiple sales lines on different documents, but not the other way.
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