‘Put it on the slate’ and ‘Wipe the slate clean’.
Back in the UK, workers used to be paid on Friday and by Monday had very little money left. So if they wanted to drink during the week they would ask the landlord to ‘put it on the slate’. The landlord would put the customer initials on the blackboard and by the side of it, the number of drinks that had been consumed. The following Friday the customer would pay his bill and the landlord would ‘wipe the slate clean’.
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So, to put something on the slate means to get credit; to wipe the slate clean means to clear one’s debts.