I think it's Arsenal and not Celtic that are being difficult here though. We have no obligation to buy if they set the price too high, sure, but they're certainly not obligated to sell him for less than what they think he's worth either.
Personally I feel we should just pay them at this stage. You don't have to accept the first quote you get, but spending months haggling over £5 million when so much is at stake if we fail to land him is not on. You have to overpay a little sometimes to get good players, and if Tierney is good enough to start for us then he's particularly appealing to me since he's also able to play centreback. We could let Monreal leave knowing that he's been replaced in full.