Hi,

This is probably you might have created a manual event or End rental event which had created a negative rental transaction. 

Go to the Rental Transactions window and observe the transactions for negative values. 

Usually negative transactions are created to adjust the previous value based on the events you have created to adjust/cut off some transactions, so that if you get the total transaction value they will show the balance amount as you wish. 

As a simple example, if you have started Rental at the beginning of the month and generated transactions ( hoping you have a monthly schedule ), then the system might generate transaction for the full month. If you then create an end rental event in the middle of the month and generated transaction, then it will create a negative transaction to cut off the unwanted period. 

If you have already created a debit invoice for the full month before you have created the end rental event and generated transaction for the second time, then when the second time if you try to invoice it will only show you the negative transaction. Here you are supposed to create a credit invoice, not a debit invoice to get the correct balance to the GL. 

Hope this helps?

AsankaD