This sounds unreasonable at first glance but it's impossible to say on the limited informatiion available. Has permission been granted yet? I think you need to start by questioning the Council's justification for an affordable housing contribution in the first place rather than looking at the "social landlord rights" issue. Much depends on the context and the specific wording of the Council's affordable housing policy. Is the proposal only being looked at favourably because there is an essential agricultural need on the farm or would the conversion to residential use have been permitted anyway? If the former then a £30k contribution could undermine the "usual" financial viability test (although there's some debate as to whether the test remains post NPPF). You don't mention whether the conversion will be subject to an ocupancy restriction. If so, in itself, that's likley to reduce it's market value and make it more affordable as well as restricting ocupancy to someone with a local need. So in the wider scheme of things you might already be viewed as providing an affordable dwelling of some description! You don't say whether this £ contribution is so great that it will put the scheme in doubt or whether the farmer just doesn't want to pay it? More background would be useful but it sounds like you need to talk to your Council to understand their position better before deciding how to approach it.
