You need to really take a look at the 2015 as to how you did this.

For example, if I bought something from you for $1,000, and paid you $200 in 2015, what did you report on the 2015 return? $1,000 or $200 on my sale.

If you reported $200 in 2015, then when I pay you $400 in 2016, that is taxable income in 2016.

If you reported $1,000 in 2015, then only interest is taxable on the money I paid in 2016 (if interest is charged).