PINES policy does not allow patrons to carry a 'credit' on their account. However, some activities may occur which results in patrons showing a negative balance on their account which should either be cleared or refunded. This allows you to look up patrons with negative balances on their accounts per library (or system):

Administration > Local Administration > Patrons with Negative Balances

Patrons may also have a combination of negative and positive bills on their account. If this occurs, the patron will only appear in this interface if their overall balance owed is negative. A report must be used to identify other negative bills. There is a Quick Report template under Bills > Bills with Negative Balances to identify bills with negative balances, regardless of whether or not the patron's overall balance is negative.

See: Reconciling Negative Bills