pur req line item wise block

Hi Gurus,

Is there a way to achieve this specific scenario. Let us say we have five line items on the Pur req.We have setup our approval system in such a way that if the dollar amount is $1000 or more then only the release startegy kicks off. Below 1000 there is no release strategy. So now out of the 5 lines lets say we have the first four line items less than 1000 and the fifth line item is $2000. we want to put a block on the PR so that unless the 5th line item gets approved, the remaining 4 line items which are less than 1000 should not be converted to a PO. How can this be achieved?

Thanks

ANusha