Is it legal for an Employer to make someone sign a Payroll Deduction form to pay an on going bill?

My girlfriend works for a vet clinic here locally. They allow workers to have accounts and they only pay for half of the procudes its a half price thing. My girlfriends bill is $370 and there is no time its due just what ever u can afford to place on it. She paid $200 in April when it was 570 something. They gave her a paper for payroll deduction and told her she had to sign it. She did not knowing what it was for and when she brought it home and i read it. It was stating that $50 a pay period will be removed for the account. now i know she signed it. Is there a way for her to change it from $50 to $10 bc 50 is to much and we have other bills that take first. Does the employer have to stop the deduction when you tell them to? or can they just do it now bc the papers signed and theres nothing we can do about it?
thank you for your time
Kyle