Sand River Convention
The Sand River Convention is a decision by Transvaal. It is possible to enact if one have researched State and Government and have a relation of at least 75 with United Kingdom. It also requires that Transvaal does not own any provinces, that has a British core. British South Africa has British cores, so one cannot have annexed South Africa. It gives a +25 better relation with United Kingdom and outlaws slavery.
The decision does not really give enough credit to the actual Sand River Convention. The convention was actually United Kingdoms formal recognition of Transvaals independence and outlawing slavery was merely a part of the convention.