new deduction available for up to $300 per taxpayer ($600 for a married couple) in annual charitable contributions. You don't have to itemize to claim the deduction, but the same rules apply for documentation

Emphasis on " the same rules apply for documentation." You actually must have made the donation in order to claim $300/$600.

Obviously, IRS is not going to audit anyone to check on a $300 donation.  And everyone knows that -- this is just the camel's nose under the tent so that the amount can be raised in the future.  

Note that the $300 contribution must be a documented cash/check donation. 

I am getting acquainted with the new 2020 program, and it won't allow $600 for a married couple's contribution, $300 is the max.

If I put  it on a return without client's documentation, I have committed tax fraud.

And if they sign the return, they have committed tax fraud.

I don't think $300 rises to the level of fraud under the Internal Revenue Code.

I do agree it would be wrong to claim a deduction that was not made and for which client does not have documentation.

Please note that @Greta has now asked this as a separate topic, here:

https://proconnect.intuit.com/community/proseries-tax-discussions/discussion/600-contribution-deduct...

$600 for a married couple filing jointly works in 2021.  I don't know what the max was in 2020; like you I had problems with it in 2020.