I assume that you are asking about the Spanish response to an American offer to buy Cuba shortly before the Spanish-American War broke out. At some point in 1897, President William McKinley of the United States wrote a letter to the Queen of Spain, offering to buy Cuba for the sum of $300 million. McKinley thought that this would be a good way in which to solve the problem of what to do about Spain’s efforts to subdue the Cuban independence movement. The Spanish government, however, did not accept the offer. Spain’s government had spent a great deal of money and effort in trying to keep Cuba from becoming independent. Therefore, they were not eager to simply turn around and sell the island that they had worked so hard to keep. This prevented the Spanish government from accepting McKinley’s offer.
Some people say that this rejection was one cause of the war with Spain. However, I tend to discount this suggestion. The rejection had nothing to do with the explosion of the Maine and the de Lome Letter was not written as part of that rejection. Furthermore, as the link below shows us, President McKinley made no mention of the offer or the rejection when he asked Congress to declare war on Spain.
So, the answer to this question is that Spain rejected the US offer to buy Cuba, but that rejection was not a major cause of the Spanish-American War.