Even one second of usage of land tier over your current level will make you bump up to the next level. That's why you can't make an overage purchase even temporarily, with the intent of fixing it -- the Lindens will not help you on this (you can petition and beg, and sometimes on a first-time newbie mistake, they will fix it as a courtesy one-off offer, but it is not their default.)
There is absolutely no reason why you cannot undertier your group temporarily to make the purchase, then purchase it, and then deed it to the group -- if you've done the math.
The reality is that there is a window of about 24 hours before you will get messages from LL about needing to add tier to your group. Indeed LL does seize land (the latest land to be added to the group) if you are undertiered. But it is a process where you get a warning first, and there is a window. It is not worth risking more than a few minutes, in my view, but I've seen it go to a day or more when I've accidently had a group under tier.
There is no stated rule about this anywhere, it is a discretionary and manual process. But you shouldn't be forced into upping your tier level, or renting tier at much higher prices, merely for a transaction like this IF you have determined that you can add the tier in through the group bonus or other residents helping you.
BTW, you can "buy for group" to instantly make use of the group bonus -- again, if the math is working out.