The gemara in gittin says that on Shabbos a Jew is allowed to buy a house form a non-Jew in Eretz Yisrael. The reason is that it is such an important thing to buy the house form a non-Jew because of the mitzva of yishuv eretz yisrael.

In practical halacha it seems to be a bit more complicated, as there are different opinions as to what is actually allowed to be done on Shabbos. It seems only rabbinic laws can be forgone in the purchase on Shabbos, such as telling a non-Jew to sign the papers. Others say that it is so important it is like a part of a milchemet mitzva and anything can be done on Shabbos to successfully acquire the house away form the non-Jew.

While not quite the same level of prohibition or allowance, Rav Elyashiv has recently paskened, according to reports on Kikar, that because the housing crisis has gotten so bad one would be allowed to buy a house during the 9 Days if the opportunity suddenly presented itself, despite the fact that it causes such great happiness.