This is a very good question. In Canada, Quebec, there is something that we call "Contre-Lettre" I believe that is also common in other countries. In order to safeguard your share and assets, you must request a document (counter-Letter) from the notary which stipulate you have ownership of 20% and you will be responsible to pay revenue generated from of this asset. If dispute may occur between you and your co-investor in the future, the law is clear: the counter letter prevails over the apparent contract.