50B is the amount they've been asked to sell. The value of Greek government assets would be in the hundreds of billions - maybe more than a trillion. Parks, land, buildings, military assets, state owned businesses - it's a big country.
So if you want to compare them with a mortgage holder it'd be more like someone with a million euro in assets, 100,000 in interest only debt and maybe an income of 25,000.
If Greece want to say their debt is unsustainable then sell some assets and make it more sustainable, looking for more cash and simultaneous write downs from the same creditors is not a sensible or fair strategy.