VA01 - Returns Referencing Billing Document - Consider all previous Returns

Question

I have a requirements that if there is and invoice for quantity 20 and the customer has returned 10 and then another 5 (therefore, leaving only 5 of the original 20 left to return), I need to populate the quantity on the next return with the 5 remaining, not the original 20 when referencing the original invoice.
I know ABAP code is going to need to be written, but I need some direction on where to do this.  I have tried the user exists and modifying the SAP code. 
Also, reactions from the system:
1)  When the user goes to create the 3rd return, the quantity on the return should be 5.
2)  The user should be able to change this to anything equal to or lower than the 5.   If they change it to 6 or more, I need an error telling them that 15 of the 20 have already been returned.
I hope this makes sense.
Thanks,
Cindy

Answer 1

Thanks for responding so quickly, but I must not have been clear about my requirement.
Here is the flow:
Original order for 20
Delivery for 20
PGI for 20
Invoice for 20
Customer returns 10
Return for 10 with reference to the original invoice
RMA - Returns Putaway for 10
GD Returns for 10
Credit for 10
Customer returns 5
Return for 5 with reference to the original invoice
RMA - Returns Putaway for 5
GD Returns for 5
Credit for 5
Then the customer wants to return the remaining 5.  When you go to VA01 and reference the original invoice, it populates the item with a quantity of 20 which is the original quantity even though the customer has already returned 15.  We want to restrict the returns on this item to no more than the original 20 even when doing several returns.
Hope that is more clear.