Mark then wrote three letters to his three main local competitors announcing he would sell his business to either one of them who offered him, in writing, the highest price. He also stated he must hear from them, if interested, before 5 pm on Monday 22^nd March. He posted these three letters on Monday 8^th March.

John received his letter on Thursday 11^th March and telephoned that night to discuss the sale with Mark. The call went through to the message bank so John left a message saying 'Call me, as I am happy to buy your business', and left his name and phone number. Mark called him back on Sunday afternoon and John specified a price at which he would be prepared to buy the business. Mark said he would think about it and get back to him.

I'm just confused whether or not Johns offer would be accepted or deemed invalid because he called him instead of writing back to him. Since Mark said it must be in writing.