(1) The seller may declare the contract avoided:

(a) if the failure by the buyer to perform any of his obligations under the contract or this Convention amounts to a fundamental breach of contract; or

(b) if the buyer does not, within the additional period of time fixed by the seller in accordance with paragraph (1) of article 63, perform his obligation to pay the price or take delivery of the goods, or if he declares that he will not do so within the period so fixed.

(2) However, in cases where the buyer has paid the price, the seller loses the right to declare the contract avoided unless he does so:

(a) in respect of late performance by the buyer, before the seller has become aware that performance has been rendered; or

(b) in respect of any breach other than late performance by the buyer, within a reasonable time:

(i) after the seller knew or ought to have known of the breach; or

(ii) after the expiration of any additional period of time fixed by the seller in accordance with paragraph (1) of article 63, or after the buyer has declared that he will not perform his obligations within such an additional period.