Usually fencing would be written into sale contract as to whose responsibility it is.
To me your trying to have you're cake and eat it at the same time. Going halves with the farmer initially seemed to be a good compromise. You didn't go that route and he covered 100% of the cost. Now you want him to go halves this time. I can see his colourful answer to that already forming in my mind.
You should cut your losses, ask him for permission to erect the fencing, get the boundary measurements right, and just erect your fence.
I put up concrete posts and chainlink fence between my neighbour and myself (having said it to him), fence up and solicitor told me that the fence is 100% mine because I 100% paid for its erection. Neighbour can never interfere with it.
Good but expensive option ... concrete posts, chainlink and strainers.
Cheaper and faster option: timber post and rail (If you go with this option stand post in compacted gravel rather than in concrete as the posts would eventually rot at their bases.)