I think that you should be careful in not overclaiming in mixing things.
It could be that there are laws in Dubai about such compensation and that is what the agent there was referring to.

If your ticket was CDG-DXB-CDG, the compensation due according to EC261 is clear. It should be 75% of the ticket price. Typically that would be 75% of half of the return ticket price if all in PE. There is no mention of a EUR150 voucher. The voucher is complicating things and I would stay clear of it. Maybe it is a Dubai thing, but that is not provided in EC261.

Your situation might be different because it is a ticket from non-EU to non-EU and only UK261 applies. The National Enforcement bodies are usually of little help. As UK resident, I suggest that a small claim court would be the way to go. But I am no legal expert.