Negative accrual balance

Summary

Need ideas on how to set up accrual plan to "borrow" against accruals for the plan year, but not be negative by end of year

Content

I am setting up a new absence plan that will have a monthly accrual for employees (changing from annual front loaded), and we are permitting negative balances so that employees can take longer vacations at the start of the year if desired (limit of -40 hours), even though they have not accrued the time yet.  However, I need to restrict so that by the end of the year they are required to have earned back the time - so, allow negative usage up to 40 hours, but not allow total usage of more than the annual accrual would be.