I borrowed money from my brother-in-law to pay bills. He paid the bills directly. Now he wants to change the agreement.

On January 8, 2018 I entered into an agreement with my ex-brother-in-law where he loaned me money to pay bills while I was on leave without pay from having my daughter. Our agreement was that he would pay my bills directly and I would pay back half the amount loaned plus interest ($3650 to be paid back including interest) once I filed/received my tax refund. The remaining amount was to be paid back by helping my sister 37 times. We agreed that I would help on weekends, but not every weekend, and occasionally during the week (I am a single mom to a 4 year old and 3 month old). We both agreed to the terms, and he immediately started paying the bills online. The bills he was unable to pay online he got cashier's checks. I started helping my sister on January 12, and have helped 10 times. On the 24th he texted me "due to recent events" he would need all the money I borrowed ($6600) back when I get my tax refund. I am unable to pay him back the full amount. What are my options?