Is Dana (donation) more about detachment from one's possessions & to break one's ego or is it more about helping others?
DANA SUTTA

In this Sutta, Buddha broke it down to 7 categories of why people give Dana 
(1) To answer your question as to purpose of dana, people give for many reasons. 
(2) some give to detach(ornament of mind) which bears great fruit and great benefit. 
(3) some people give to help others. 

I think you can answer yourself if the reason you want the merchant to maximize his profit is because you want to help him out or you are doing it to let go of your attachment.