The agreement includes urbanistic planning around the temple. One of the major issues is that to complete Gaudi's original design, there should be gardens and

The agreement includes urbanistic planning around the temple. One of the major issues is that to complete Gaudi's original design, there should be gardens and accesses that are now occupied by buildings. Since the Sagrada Familia is not officially being built (because it has no permit), the city cannot officially study (if it can / how to) free that space. Now they will be able to make these studies, and will have a proper justification if they end up deciding to expropriate some buildings to make space for these gardens/accesses. That's the real reason why the Sagrada Familia managers agreed to the payment... Edit: Also, the Sagrada Familia is already exempt from paying property (not construction!) taxes because (a) it is owned by the church (religious temples don't pay property taxes in Spain); and (b) it is a good of public interest (that also don't pay taxes, like the Gaudi's designed Casa Batlló for instance). That is, "the leftists" could go after a much larger pie, and we must assume they don't because they somewhat agree with your reasoning.