You are correct that TRUE 100% cash basis businesses do not carry operating liabilities (such as monthly vendor billing) or even accounts receivable (customer invoicing) but this cash basis refers to revenue and expenses and does not preclude the realization of loan liabilities.
If you do carry A/R and A/P on your books you are using a recognized and accepted hybrid accounting method, embraced and allowed by the IRS (as long as you are consistent year to year)
If this were simple A/P you would enter it as a customer credit memo and by refunding it over time your expense (or contra income) in the cm would be realized over time. Simple as creating a partial refund annually. Despite it not being recorded as an actual loan it will hit your balance sheet just the same.
If you would rather treat it as a loan to the business you would record the loan in full against the change in revenue - which would give you full expense today rather tan accrued over 5 years.