On the "Balance ledger" step of the 1VALET Payments setup, you will have the option to allow residents to underpay their balance. You can see a full video walkthrough of the process here.

When enabled, this will allow a resident to pay less than their balance owing. For example, if their balance is $1000, a resident will be able to make a partial payment of $500 towards the balance. This is helpful if you want to allow multiple residents in the suite to split the balance owed.
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If you would like to prevent residents from paying less than their full balance, you should not enable this setting.