Posting Partial payment for Invoice and manually clearing the Items

item 1 Balance of 1st bill-24000

Item 2 Incoming payment - 20000

Item 3 Adjustment - 2000

These 3 items appears as open items in the Vendor Line item display. But when i try to clear the vendor account manually using F-32. The item 1 should clear against the Item 2 and create a new residual item - 4000. But when i try to clear the vendor account i am unable to it says the difference is too large for clearning...am i missing some thing to perform. Or any body can suggest me the right way to clear and what items to select.

Looking farward for your earlier reply.

Best Regards

raj