The 35-year-old singer shared a video on Instagram showing her rehearsing with her 70-year-old husband.
In the video, McPhee belted out a tune while Foster offered advice.
“She’s making fun of my song,” Foster said to the camera after interrupting McPhee’s singing.
“You’re using me for my talents,” the songstress quipped.
“Totally,” joked Foster.
As the video continued, McPhee said that while performing, she’ll have to make sure to keep the melody of Foster’s song intact, otherwise she’ll “have to take five percent.”
“You already got 50 percent,” said Foster. “Didn’t include theatrical releases though.”
After a laugh, McPhee turned to the camera, smiling and said, “It’s called a prenup.”
The two were preparing for Foster’s upcoming tour, on which McPhee is set to appear as a special guest.
“We are coming to a city near you starting next week! Come see us, I swear we’re fun,” the “American Idol” alum wrote in the video’s caption.