Of course, the Senate can not originate a spending bill, only amend one that comes from the House.
Voting to amend a defeated bill does not cause it to come back up for a vote in the House - it remains a defeated bill.
They are using a loophole- http://money.cnn.com/2008/09/30/news/economy/bailout_tuesday/index.htm?cnn=yes
“Because the bill must originate in the House, the Senate is attaching the rescue plan to a bill that deals with renewable energy tax incentives. This would allow the Senate to vote before the House.”