From Longman Business Dictionary
provisopro·vi·so /prəˈvaɪzəʊ-zoʊ/ noun [countableC]1LAW a part of a contract that covers a particular subjectLandlords frequently include a proviso stating that nothing contained in the lease shall prevent them from carrying out work on their property.
2formal something that must happen before you will agree to somethingproviso thatThe only proviso is that you should let the office manager know well in advance if you decide to take a day off.
Some companies are happy to invest in community projects, with the proviso that the government does not neglect its own duty.