Investment addendum not signed in a timely manner

[William Lokar]

A seller signed our contract and the sellers agent bound the contract without signing the attached Investment property addendum certifying rents (that was written in the contract to be part of the contract).   They signed the addendum 5 days later after much back and forth on what rents actually were.   To me the contract was not valid until the addendum was signed.   What is the true legally binding date?  Any thoughts on this or has anyone dealt with a similar issue?