In 2015 I inadvertendly failed to include some taxable income on my return. As a result, my refund was $700 more than it should have been and I am not re-paying on a payment plan. How does that impact my 2017 return?

If you have an unpaid balance on your installment plan, it will be offset against any refund due you.  It doesn't matter that you may be current on installment payments.  (This is what you agreed to when you signed up for the plan.)

If you have not yet  amended the 2015 return to make the income correction then do so asap.

I think it's safe to assume that if he is repaying the $700 on an installment plan, the IRS has already assessed the deficiency.  If the assessment is correct an amended return would serve no purpose.

It wasn't clear ... the CP 2000 notices for a 2015 return have not gone out yet.

Please note: I meant to say I am NOW making payments on a payment plan.