Hello,
basically all your partial payment will appear as credit item in your customer account in FBL5N. You can filter the line item on the basis of these debit items. Then as Ajay has correctly mentioned,REBZG or rather the Invoice ref field,it will contain the reference of the original document number (Refer screen shot). You can try and bring this field in the general list of line items (Refer the below path), and then start filtering them. accordingly.
IMG-FIn accounting>Ar & AP>Customer accounts>Line items>display line items>Define additional fields for Line item display
If you are an end user then probably you might require some assistance from your FI support team.
best regds
Subha