In this case it gives the Saudis central bank money, which then leads to more money being removed over the coming years. Since the Saudis want to dump it, they have to sell it below market value. Due to that at intresst this means the ECB is moving money out of the monetary system. Hence this is deflationary. We actually saw that in the real world as well. QE meant low intresst rates. The intresst rates shot up as soon as QE was stopped.