Peroutka, Stephen G Peroutka & Peroutka BACKS OUT OF AGREEMENT Pasadena Maryland

A few weeks ago after returning to work from knee surgery, I received a call from this law firm/ collection agency asking me to settle a debt that I have with them.  I had been out of work for 2 months, and explained that I get paid every 2 weeks and would be able to make payments on my pay weeks, between $75.00 and $100.00 each pay.  Erica said that would be fine.  On my first return pay, it was short becasue it was not a full paycheck.  I immediately called and expalined that and Erica said that that i could make my first payment on July 12th which was my first full pay back to work and I made my payment that morning and got my confirmation.  Today I received in the mail a letter where our district court issued a writ of garnishment to my employer.  I made an agreement to pay, made my first payment and now they are going to garnish my pay.  Then what was the point of harrasing me at work and making an arrangement?  They have no proof that it is my debt and when I ask for proof they never seem to have it, can someone tell me what to do?