A rent credit is something people have asked for, and so it is now available, althought it has always been available, well sort of.
In 8.2 you can now create a negative extra charge. This is equivilent to a rent credit. In previous versions you could have manually editted the rent periods to reduce the rent owing, and that is still available. However a negative extra charge has the advantage of showing up on the tenants statement, so the tenant can see why the total balance owing has been reduced. It also creates a better audit trail as it also shows up on the NZ T41 Rent Summary and Tenant History reports.
You can not pay off a negative extra charge, although it can be used to offset another positive extra charge for the same rent period. If the total extra charge for a period is negative, the paid amount will always be zero for that same rent period.