I just came across this today. I have 4 minions. I bought some BA, reducing my available money to $1. My minions HP were:
20/20
20/20
123/227
544/548
This requires $5 to heal (was $2 originally and had the same error, but I trained the third minions HP in the hopes that a training reset would fix the error) I was confident that I could still win my next fight, but when I clicked the fight button, I received the error about not having enough money to heal all of my minions.
Is it intentional that we cannot fight if we cannot pay for a full heal?