My advice is to cancel your appointment. Don Cargill is a charlatan, as are all polygraph operators. Polygraph "testing" has no scientific basis, and it's certainly no basis for establishing trust in a relationship.

It sounds like you partner already assumes you're guilty. You can share with him or her the information available on this website, especially that found in our book, The Lie Behind the Lie Detector:

https://antipolygraph.org/lie-behind-the-lie-detector.pdf

Polygraphs may be suitable as a ratings gimmick for trash TV (Don Cargill performs for the tawdry Trisha show), but it would indeed be foolish of you to throw your money down the polygraph rat hole.

Let's assume you take the polygraph and pass. How long do you think it will take for your partner to Google "polygraph" and discover that the polygraph can be fooled using simple countermeasures that polygraph operators cannot detect?

You (and your partner) will find those countermeasures explained in Chapter 4 of The Lie Behind the Lie Detector.

A charlatan is "a person falsely claiming to have a special knowledge or skill; a fraud" (New Oxford American Dictionary).

Polygraph operators falsely claim that they can detect lies (or deception, there's really no meaningful difference). Polygraphy has no grounding in the scientific method, which is not surprising, considering that it was devised by interrogators (Leonarde Keeler, John Reid, and Cleve Backster), not scientists. You cannot detect lies or deception. What you do for a living is a fraud.