or they could even restrict options to typical government spending.

JoshuaFox noted that the government might tack on such restrictions

That said, it's not so clear where the borders of such restrictions would be. Obviously you could choose to allocate the money to the big budget items, like healthcare or the military. But there are many smaller things that the government also pays for.

For example, the government maintains parks. Under this scheme, could I use my tax money to pay for the improvement of the park next to my house? After all, it's one of the many things that tax money often works towards. But if you answer affirmatively, then what if I work for some institutute that gets government funding? Could I increase the size of the government grants we get? After all, I always wanted a bigger budget...

Or what if I'm a government employee? Could I give my money to the part of government spending that is assigned as my salary?

I suppose the whole question is one of specificity. Am I allowed to give my money to a specific park, or do I have to give it to parks in general? Can I give it to a specific government employee, or do I have to give it to the salary budget of the department that employs that employee? Or do I have to give it to that department "as is", with no restrictions on what it is spent on?

The more specitivity you add, the more abusable it is, and the more you take away, the closer it becomes to the current system. In fact, the current system is merely this exact proposal, with the specificity dial turned down to the minimum.

Think about the continuum between what we have now and the free market (where you can control exactly where your money goes), and it becomes fairly clear that the only points which have a good reason to be used are the two extreme ends. If you advocate a point in the middle, you'll have a hard time justifying the choice of that particular point, as opposed to one further up or down.