Gifts to Reduce the Public Debt

I wonder if it would make any difference if the giver could specify how the donation is to be used? For example, say you supported student debt forgiveness and your donation could only be used for that purpose. Would that make donating to the government more attractive?

Unfortunately, such a scheme won't work because money is fungible and donations to a specific cause would likely result in congress allocating less money to that area of the budget.

Why would anyone do that?