Hello,
for all your queries, FBL5N is the answer. When you partially collect a payment in SAP, the system creates a separate credit line item (like instalments) along with the original receivable item. Once you are done with collecting the entire payment, system will clear out the original receivable that was created out of billing. All these will be visible once you run FBL5N.
Also you can explore some other reports like S_ALR_87009956 .
Also you can refer to this link, it might answer your query.
How to retrieve partial payments made by customer
best regds
Subha