To bill an inadequate amount.

What does Underbilled mean?

Underbilling occurs when work you've completed hasn't been billed or costs you've incurred haven't been reimbursed. ... From an accounting standpoint, underbilling is the cost and profit earned on a lump-sum construction project that has been incurred within a billing cycle but has not been billed.

What does over bill mean?

transitive + intransitive. : to submit a bill (see bill entry 4 sense 4a) of charges to someone for an amount in excess of what is due : to bill for an excessive amount The contractor overbilled us for the work. Thousands of customers were overbilled.