This seems to be in contradiction with the first amendment
That's not what "shall not be questioned" means. It's a term of art meaning that the Government cannot refuse to pay valid claims on its debt.
Read it not as 'may not be', but 'nothing shall be done that should give cause to'.
That's not what "shall not be questioned" means. It's a term of art meaning that the Government cannot refuse to pay valid claims on its debt.