Firstly, in case you need information on How to Imput a Expense, click here: http://help.rentlysoft.com/en/articles/5367312-how-to-make-a-new-expense
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Secondly, enter a specific booking, and select the "+" symbol below the Price and Deposit/Guarantee on the right side of the booking screen.
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Complete the data of the Expense and click on Pay.
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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 add Expense" permissions