Is it permissible to benefit from foreign money found inside used furniture that was purchased, given that the person who found it lives within the boundaries of the Haram in Makkah?

The foreign currency you found, which does not belong to you, is treated like a Luqatah (lost and found item). It must be publicized (identified) for a full lunar year. If its owners appear, it must be handed over to them. Otherwise, you may benefit from it after the year has passed, but you must pay it back to them whenever they are found. The first person to be asked about it is the original seller, as it is possible the money belongs to him.

If it is impossible to identify its owners, then either the finder may take ownership of it without publicizing it, according to one opinion, or it should be spent on the welfare of Muslims, and this latter opinion is the more probable. There is no difference in the ruling of Luqatah between the Haram (sanctuary of Mecca) and elsewhere, nor between a pilgrim and others. It is permissible to pick it up, with the obligation to publicize it for a full year before taking ownership. Immediate publicizing of the Luqatah is not a condition; rather, it can be publicized at any time during the year, though immediate commencement is better.