ex employer changed oral agreement

[Soupysweetie]

We bought our house in 2005 and my husbands employer offered him to put our lumber purchases on their business account so we wouldn't have to come up with a large sum of money all at once. Then they let him charge plumbing parts from them to re-do some things in our house. My husband told his employer to take $25.00/week from his paycheck, she started taking $100.00/week out so he went to her office and told her he could not afford for her to do that and to only take out $25.00/week and if we could pay more we would. They layed him off a few months later and so we dropped the payments to $25.00/month because it was all we could afford. We have never missed a payment so far. In January of 2007 she decided to start charging interest and now has raised the interest in jan of 2008 and notified me today that she has done this. We had only an oral agreement as to what we were going to pay and have always made a payment every single month. Can she now charge interest on these monies owed?