If you had the original terms in writing, this would be easy for you: you could hold them to their contractual agreement and sue for breach of contract if they don't honor those terms, with a very good chance of success. Without the terms in writing, it is more difficult, but not impossible--there are several grounds under which you could try to hold them to the original terms, such as:* Breach of an oral contract;* Fraud--deliberately lying to get you to sign up with them;* Promissory estoppel--going back on a reasonable promise they made specifically to get you to hire them to do the work.But without written evidence of the original terms, winning is not guaranteed, since they may be able to convince the court (if you sued to enforce the terms; or they sued because you were not paying the way they now evidently want you to) that 12-months interest free was just something maybe discussed at one point, but never agreed to or promised. It probaby would be best to try to negotiate with them and settle for something you can both live with--maybe 8 or 9 months interest free--rather than heading to litigation.