Select the booking in reference
In the detail of price look for the items to eliminate
By eliminating the items of the canceled booking, whatever was paid from the client will figure as balance in favor for future bookings.
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If you make a new booking with that client, that has balance in favor, you may choose to use that balance in favor:
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In case you do not have permission to proceed with this transaction, inform your administrator so he can enable you the "Can remove booking price item " permission.
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In case you need more information about how to use the balance in favor of a client, http://help.rentlysoft.com/en/articles/5357121-how-to-use-a-balance-in-favor-of-a-client