Why do you say that? If the rebbe is interested in something from a judaica store, he'll use your cash for it. If he wants to pay his grocery bill, he'll be happy you have him cash instead of a gift certificate.
Giving petty cash just isn't classy, IMHO.
You can always get away with giving a GC. Now, lets say you'll need to give $180 cash for him to be satisfied with the gift, if you give him a $180 GC to somewhere it will seem a bit over the top.
Basically, when giving a Gift Certificate you can get away with giving smaller amounts. So now that you only want to give $50, it should be a Gift Certificate.