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Hello, my name is Jim and I am a qualified lawyer happy to help you this evening.
The fact the fence is on your side means that technically it belongs to you - I do not see how he can say you are liable for a fence which he erected and which is also on your side of the boundary?. The deeds and plans would normally show the boundary lines and would also show either joint responsibility for the fence or one party is responsible for the upkeep and any repairs. If the fence is a joint one, it should be shared (the cost) however if the previous fence was not in disrepair then you can argue you should not have to pay for this one - you can suggest a surveyor's report to resolve the dispute (and go halves on the cost) which may save any court action and future unpleasantness.
It seems to me he has reneged on the agreement to pay for the fence himself - and blackmailing you with court action if you do not agree to a wall which will cover more of your land.
The other option is mediation - you can find a local mediator here :
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