Blanket Agreement Release - ability to choose line

Hello - I would appreciate any help on this.  We have a couple blanket purchase agreements with the same item on two different lines.  The buyer wants to release off of line 1, and when line 1 is fully released, release off of line 2.  Problem is, oracle doesn't give the user to chose of lines, it just automatically releases from line 1, even though the quantity agreed on that line has already been satisfied and makes the available amount to release go negative on line 1.  I searched the knowledge database here and couldn't find anything.  when releasing off