Another way of thinking about it might be that he's getting his pleading in early before the next CSR.
You can probably guarantee that one Treasury line will be : "Mr Fallon - why should you avoid cuts when you're sitting on a pile of underused property assets with realisable value?"
As Leon quite rightly points out, the amount realised since SDSR is £380m, which works out as one or two new Typhoons per annum. More to the point, the big value items have already gone, leaving sites potentially requiring big remediation - unlikely to realise huge net amounts.