No need to enter a "negative amount" anywhere in the software. When you amend, enter the newly amended amount of it being non-taxable by changing the prior positive addition to income to zero. The Amended return will report zero as opposed to any prior taxable amount.
The 1040X will report the original amount in Col A (as an example, 5000), your correct amount will report in Col C (in this case, zero), and the middle Col B will be generated as the reduction difference or -5000. No one enters a "negative" amount anywhere in the software, rather the software accommodates the auto-reduction by posting it in Col B.
Cheers.
