I discovered a solution on the receivables side --

I accidentally imported a sales invoice with a date in a closed period. To correct, I simply imported a reversal of the original invoice, with the same incorrect date. 

Then I was able to 'receive money'  on both of the invoices by selected both outstanding invoices, cash was unaffected.

I then imported the correct date on a new invoice and my AR balance is correct, and neither of the invoices in the closed period show as outstanding.

I discovered a solution on the receivables side --

I accidentally imported a sales invoice with a date in a closed period. To correct, I simply imported a reversal of the original invoice, with the same incorrect date. 

Then I was able to 'receive money'  on both of the invoices by selected both outstanding invoices, cash was unaffected.

I then imported the correct date on a new invoice and my AR balance is correct, and neither of the invoices in the closed period show as outstanding.