Is it a deal? or Have we got a deal? "A" deal. Then, later, if you are discussing the matter (asunto), you would refer to the deal as "THE" deal: What is the deal you made with Jaime? The same as in Spanish: "¿Tenemos UN trato?" and (depues) "EL trato que hicimos dispone que...."

Do we have a deal? Agreed?

That should be 'a deal'