Common property is common property. If the owner of the unit is cashing in on the extra rent, that is illegal. If the owner is prepared to pay for that parcel of land, so that parking spot becomes part of the owner’s unit, then that’s fine. The owner will have to approach the committee and ask if he can pay for the land the vehicle is parked.
Otherwise, it’s common property, and the committee can inform the owner it’s illegal, or if you make an agreement, all the rent received from the tenant for that parking spot, it should be forwarded to the Strata Bank Account.
Get everything in writing as well.