To me, this seems to be suggesting that £400k is what we will be getting compensation for him in the summer if he goes, so we are looking for a bit more than that now if anyone is interested.
Think Brown would be doing this for one of two reasons. Either to put him in the market now, and try and raised some cash. Or to put him in the market to try and show that no-one is willing to pay that now to encourage him to sign a new deal.
Personally, unless there is a decent amount over the predicted compensation offered this month, I don't see the point in selling him. The January market is over-inflated, so Brown won't be spending the money now (it may even disappear and be forgotten about by summer). Not only that, but every signing so far has been short-term contracts as Brown wants to rebuild in the summer.