A Chairman, like all other Councillors has no right to act as an individual and therefore your Chairman has no right to speak to the farmer about this matter. The disposal of an asset is a matter for Full Council, as is how the proceeds are split. The challenge here is that 3 parishes own the property and therefore it would make sense to have a cross parish committee to deal with the disposal. As for the splitting of the proceeds, again it would be down to all the parishes with a vested interest to decide - it could be split proportionately depending on the amount of money the parish has spent. Also bear in mind that you will also need to decide on the how the costs associated with the sale are split too....
But first and foremost, the Chairman needs to be shown the Local Government Act 1972 where it expressly states that he is overstepping the mark.
Good luck with this one - looks like a minefield!