Full Synchronisation Requirement - Steal the painting without the merchant spotting you
Head to the bazaar and use Eagle Sense to scan the potential thieves. When you find the right one, attack him and try to land a counter on him to bring him down quickly. When you find out where the merchant is from him, head over there and take a look at the situation. You can try to use a bomb to distract him, or alternatively hire the Romanies a little to the south of his location. The painting is to his right on the stall; interact with it to steal it back when blended with the Romanies and you should be able to get away with it scot free.
Then, simply bring the painting back to Sofia at the objective marker to end the memory.