I have a debt that I agreed to pay $50/per month on the 30th of each monthwith one of the collection agency's reps. I've made one payment & was attempting to make another, when I got a call from the legal department saying that $50/per month was not enough. Can they do this once a payment plan has been agreed to?

Continue to make the payment agreed too. If they refuse payment or return it to you that would place them in a position of dishonor.

But you should have got that in writing