Is it Constitutional to give Israel financial aid?

I would protect Israel so much, they'd be so safe it would make your head spin. But the OP is taking some serious incoming! Very randfesive!

Why single out Israel? 

I may be simpleminded but, imo no foreign country should receive aid from us.  If a foreign country requests aid in the form of weapons or our military's presence or whatever, then it should be debated in Congress to determine whether or not such aid is best for our own country's (and most especially for our own citizens') interests.  If yes, then payment from that foreign country to cover the costs of such aid must be required.

On the other hand, if aid is requested for the citizens of a foreign country because of some real disaster (i.e. earthquake, tsunami, etc.), then some sort of fund should be set up for that instance (and ONLY for THAT instance) where individuals may donate money, food, clothing, medicines or whatever,  if they choose to do so.  The American people used to be the most generous people in the world, until their own government left them so strapped that many can no longer afford to be so generous anymore.