It is sometimes helpful to prevent an account from going below a certain threshold balance. The save directive allows you to specify a minimum balance for an account, which will be deduced from the account available balance for the transaction. 
// Prevent the end balance foo is never less than [USD/2 100]
save [USD/2 100] from @merchants:1234

send [USD/2 500] (
  source = @merchants:1234
  destination = @payouts:T1891G
)
In this transaction example, even if the account @merchants:1234 has an initial balance of [USD/2 500], the transaction will fail as the account post-transaction balance would otherwise be less than [USD/2 100]. Should additional source of funds is provided, the account will act as if its balance is [USD/2 100] less than it actually is: 
// Prevent the end balance foo is never less than [USD/2 100]
save [USD/2 100] from @merchants:1234

send [USD/2 500] (
  source = {
    @merchants:1234
    @world
  }
  destination = @payouts:T1891G
)

[
  {
    "source": "merchants:1234",
    "destination": "payouts:T1891G",
    "amount": 400,
    "asset": "USD/2"
  },
  {
    "source": "world",
    "destination": "payouts:T1891G",
    "amount": 100,
    "asset": "USD/2"
  }
]