Incomplete payment of plot that led to resale.

I had a plot and I wanted to go for further studies then I agreed with my sister-in-law to pay in installments as I continued with my studies, she was supposed to pay me 3,000 USD but she paid me only 2,400usd when I was last semester I contacted her for the balance she wouldn't pick up the calls I decided to sale the plot to another person with the hope of refunding her. After sailing I made every effort to meet her so that I make an arrangement to start to pay her but she would not avail her presence to me. Not until recently that she write me a message that she paid me a sum of 3,400 USD and I must return it all now and if I don't she is taking me to prison. My concerns are; 1. We never had a written agreement with her it was a verbal agreement. 2. I didn't deny refunding her money but the figure she mentioned wasn't realistic. 3. The money she sent was through MTN mobile money Ug i lost that line and I can't trace it for evidence as well as for her she has no evidence for the money transaction. I need some advice, please.