When clients make a payment, there is a field to set an amount for credit that will not be applied (at time of payment) to any invoices and result in an account credit.

Some do not want clients to be able to make a payment for credit, and if they can, to be able to set a minimum and maximum amount of credit.

This should be a company, client group, and client setting.

Settings > Company > Billing/Payment > Credit Handling (New link), and a new section under Settings > Company > Client Groups: Edit called "Credit Handling", and section under the client profile page: Edit for "Credit Handling".

[x] Allow Client Payment for Credit (Enabled by default) (If checked, allow a minimum and maximum value to be set for 1 or more currencies)
Minimum Amount (Input), Maximum Amount (Input), Currency (Dropdown) (Null by default, meaning no limits set)

If there is no setting for the currency, then there would be no limit.

The minimum amount would apply to every payment for credit. The maximum amount would apply to the maximum amount of the current payment, OR the combined total of the current payment + existing credit, whichever is lower. For example, if the max is $5000 USD, and the client has a credit of $2000 USD currently, they would only be able to pay $3000 USD.

When a client is making a payment, if they are not allowed to pay for credit, do not show the payment for credit field. If they are allowed to show the payment for credit field, and a minimum or maximum value is set for the currency, show the minimum and maximum value next to the field. e.g. ($50.00 USD minimum, $5000.00 USD maximum) showing the minimum payment and the maximum allowed, the lower of maximum setting, or maximum setting - existing credits. If the maximum is equal or less than the minimum, show a message like "Not Available" (message open for discussion)