Michaella enters into a contract with her tax accountant, Andrew, to prepare her business tax return within four weeks of the end of the tax year. The contract provides that Andrew is to receive $1,500 for his work and specifies that time is "of the essence". Three weeks after the end of the tax year, Andrew calls Michaella saying that there may be delays in processing the tax return. He demands an extra $500 to make sure that the return is submitted on time. Michaella agrees. The tax return is finished on time and she receives her tax refund. She sends Andrew a cheque for $1,500. Andrew rings her to remind her about the extra $500 "as agreed". Is Andrew legally entitled to the $500?