1. If the 2 BTC worth more than before (let's assume 1 BTC worth 600 USD), the employer is certainly not going to pay a salary higher than he promised.
2. If the 2 BTC worth less than before (let's assume 1 BTC worth 400 USD), the employee is certainly not going to accept a salary lesser than was promised.
Well since they both agreed to that they are responsible to honor the deal taking into account all risks.
Actually only the employee takes some risk. If the employer buys the coins when he makes the deal then he will be a crook if he decides to sell them instead of paying the employee the day of the payments.