I can't speak to every dealer but when I was shopping, the Mahindra dealer was very eager to deal on price and extras.
On the trade, you will always get a better price if you sell it yourself. You will want to weigh that vs the ease of trading it off and make your own decision on what's best for you. I don't know anything about your tractor but the trade price seems low to me.
I would check with Mahindra on the compatibility of a backhoe for your tractor. If the same dealer that isn't budging on price and offering you an exceptionally low trade value is also telling you you can't get a BH for your current tractor, I would be suspicious of his motives. If a BH can be added, I would find another dealer to buy it from.
Good luck.
John