Ask YC: Share purchase reversal?

I worked for a company that gave me 5% in equity stock. I didn't like the way things were going at the company so I decided to sell my stock and move on.

A guy agreed to purchase 1% of my shares, and has paid 62.5% of the agreed amount.

Throughout last year and this year I called him and asked when he would be paying the balance.

In March he said that the company is not going too well, and he doesn't have any money to pay now.

Today I got a call from him where he told me that he wants to void the contract. Does this mean that he wants his money back? Is this legal? This is 11 months after the original agreement.