Inclusive free goods determination

Dear Gurus,

My requirement is like I have created inclusive free goods det. order with qty 90+10 , delivery is done however while invoicing I created invoice for tan item -90 and tann item 10 separately i.e two invoices. Upto this it is ok. However while creating returns sales order w.r.t billing docs the system is disabling the second line item. Following are the my requirements:

1.I want to enable the second line item i.e TANN , currently it shows in disable mode bcoz of inclusive free goods with item generation. As a result of disable , I am unable to change the free goods return qty.

2. Can I have a check that if main item is returned so proportionately the free goods should be determined in second line item.

Please advice