Not sure that a used car, in that part of the world, not from an authorized dealer, gives the OP much recourse. If the seller was willing to lie on the mileage, why would he take it back?

Is it a gray-market import? If so, check for water damage as well as collision damage. Lots of US vehicles were exported after they went wading in Katrina.

KM or miles? If km, then 100,000 isn't so high. If miles, I would sell it now. If km, or if you decide to keep it anyway, get the authorized dealer to do a full service (assuming you know and trust them). Change all the fluids. Compression test. Get a readout of any fault codes. And do a full inspection for any damage, including getting underneath it on a hoist. On second thought, do that step first, as it may change your decision on keeping it.

Then drive it and enjoy it.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue