te $100K is taxable income in 2018 since that is the accounting method used by it. the extra $20K would go on schedule M-1 line 2
in 2019 the $20K would go on M-1 line 5
the reason is that even individuals that uses the cash basis must report certain income on the accrual basis. some partnerships/llcs and corporations are required to use the accrual basis. thus the individuals (owners) would report on their 1040's as shown on the k-1 despite different accounting methods.
look at it this way. the tax laws allow contributions to retirement plans (and certain other items) for year X to be made in the following year yet get the tax deduction in year X. this is in effect an accrual method.
