SS is a pay as you go system. Today's workers pay the benefits for today's retirees. SS has been in the red (benefit payments exceed revenue) since May 2009, which is why SS is cashing in its IOUs in the SSTF to make up the shortfall.
Obama's 2% reduction in the payroll tax just decreases revenue and increases the shortfall. And the General Fund must borrow the money to redeem the SS IOUs, thereby increasing the deficit. Moreover, the law that authorized the 2% reduction says that the SSTF must be reimbursed for the loss of revenue caused by that reduction at some future date. In essence, we are using the SSTF IOUs as a stimulus.