Border of Germany to Amsterdam train using a German Rail Pass

Are you sure you even need a German pass? What is your schedule? Rail passes of any nature are not an automatic good deal anymore. Some are but most are not when you consider the additional fees charge for reservations and deep discounts that are often available for advance purchase.

Yes, I barely need a pass. I want the flexibility to hop on when I am ready to do so

And where are you going from Amsterdam that is in Germany?

You will see why we can't give good advice yet, if you look at our host's comments on this page
https://www.ricksteves.com/travel-tips/transportation/trains/should-i-get-a-rail-pass
and in the vicinity. He has lots of train advice pages. If you look at his (deliberately) simplified rail map for the Netherlands, you will see that there are at least four major routes to various sections of Germany, from Amsterdam. Your question is not answerable without knowing where you are going.

You can use either the DB or the Netherlands rail company sites to learn the first station after entering Germany. Most rail tickets are cheaper when bought about two or three months in advance, and they can be very cheap indeed. But then they are not chageable or refundable. You have to decide what is best for your travel plans.

You also have to learn about levels of train service (Thalys, IC, etc., not talking about First and Second class) so you can trade off time against cost and convenience.

To Amsterdam on 27 September. Will be returning on 30 September from Amsterdam to the Frankfurt airport. I need flexibility due to time as we are not sure what we will be doing in Heidelberg and Amsterdam. A four day pass is the most I would need. Will be traveling a bit in Germany prior to going to Amsterdam so two days in Germany then Heidelberg to Amsterdam and Amsterdam to Frankfurt

You can get a fixed booking at www.bahn.com of 79.80 EUR for 2 1st class tickets from Oberhausen Hbf (the last stop in Germany if you use the ICE from Cologne) providing you take the 8:13 S-Bahn to Mannheim, then change to the 8:36 ICE to Cologne, which connects to the 10:41 ICE to Amsterdam. That way, you would book the 11:34 train from Oberhausen. That way, you don't get out of your seat, just switch to the ticket you bought.

Similar strategy for Amsterdam to Frankfurt Airport. Buy the ticket for the ICE ahead of time to Oberhausen and stay on the train to Cologne then continue on to Frankfurt Airport.

Using the direct ICE service to and from Frankfurt, the border station is Emmerich. A ticket for the Dutch part of the trip costs around EUR 23, and can be bought with NS, the Dutch railways. Unless I am very mistaken, these tickets are valid on all trains between Amsterdam and Emmerich (and v.v.) on your intended travelling days, giving the flexibility you apparently want.
You must do the arithmetic whether this is a better deal than a two-country rail pass.

I need flexibility due to time as we are not sure what we will be doing in Heidelberg and Amsterdam.

Bit of a chimera, that flexibility. There are 3 direct trains on Sept 30 to Frankfurt airport, 10:40, 12:40, 14:40. The 12:40 can be had now for 160 EUR for 2 1st class tickets, the other two for 200 EUR.

The prices I gave before were for advance bought tickets. If you truly want to maintain your "flexibility", you need Flexpreis tickets and they are quite a bit more, 171 EUR for 2 1st class Amsterdam to Oberhausen.

A 4-day 1st class Twin GRP is 430 EUR. Plus the 2 Flexpreis tickets at 342 EUR, and we are at 772 EUR.

On the other hand, with a little bit of commitment, you can get the Heidelberg to Amsterdam for 240 EUR for 2 1st class, and Amsterdam to Frankfurt airport for 160 EUR.

That leaves those other 2 trips. Frankfurt airport (?) to somewhere, then somewhere to Heidelberg. If it is close by, then 1st class tickets will not be that much. I see that if you can do it for less than 372 EUR, you're ahead of the game.

Edit - If you do as ton suggests, certainly the cheapest way to go, you will have 2 extra train changes, one at Arnhem Centraal, and the other at Duesseldorf. If you want 1st class, it is 39 EUR per person.

My suggestion is only about buying tickets for the Dutch part of the trip instead of buying a two-country pass. Buying tickets to and from Emmerich still allows you to take the direct ICE train: the ticket price for Amsterdam-Emmerich is the same, irrespective of the type of train you take.

Good to know that you can buy a ticket to a station the train does not stop at.

These are tickets for a connection to Emmerich. IC or ICE to/from Arnhem and a regional train to Emmerich. There is no other way to pay for the Dutch part of the journey. The alternative is to buy a ticket from Amsterdam to the first German station where the IC/ICE stops: Oberhausen.