The overdraft directive lets you instruct the Numscript interpreter that an account post-transaction balance is allowed to be less than zero.

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Unbounded

The overdraft directive allows for the account to go below zero without any limit, by using the unbounded keyword: 
send [USD/2 100] (
  source = @foo allowing unbounded overdraft
  destination = @bar
)

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Bounded

The exact amount below zero at which the account is allowed to go can be specified by using the up to keyword with a monetary value: 
send [USD/2 100] (
  source = @foo allowing overdraft up to [USD/2 50]
  destination = @bar
)