A valid contract requires both parties to receive consideration - that is some benefit that they receive in exchange for entering into the contract. In the employment law context, this means that an employment contract will not be valid or enforceable if it is entered into after the employee commences employment and does not provide for the employee to receive some benefit that they were not already receiving.
In Quesnelle v. Camus Hydronics Ltd. (2022 ONSC 6156) Justice Charney was faced with the following termination clause:
There has been some uncertainty about whether a purchaser who enters into a contract but fails to pay the deposit still forfeits the deposit for repudiating the contract. The British Columbia Court of Appeal recently answered that question in Argo Ventures Inc. v. Choi, 2020 BCCA 17.