Complex sentence: He was acting as the agent for Luis de Haro, who had planned to present it to Philip IV of Spain, but when the painting arrived in Madrid Velazquez challenged its attribution to Correggio and Luis decided to keep it for himself.
Split into simpler sentences: He was acting as the agent for Luis de Haro, who had planned to present it to Philip IV of Spain. However, when the painting arrived in Madrid Velazquez challenged its attribution to Correggio and Luis decided to keep it for himself.