Here are some ideas. I am assuming you are dancing the woman's part - if not, please ignore this.
1) Try dancing on your own with your arm toned, but you hand limp
2) Try dancing on your own, holding a raw egg in your right hand
3) Instead of crossing your thumb over your partner's you could try the English-style parallel thumbs hold. At least that way you wouldn't be able to squeeze as hard
4) Instead of putting more tone in your arm, you could work on making sure your upper arm functions as an extension of your shoulder. You do not really need tone in the arm, rather you need your body to respond to the way your partner moves you hand. But this difference will take weeks, months, maybe years to understand.