Remove the Credit Button when the Bill is Fully Credited (Paid in Full)

Create a Workflow that will remove the Credit Button when the Bill status is Paid in Full.

Navigate to Customization > Workflow > Workflows > New.

Workflow setup as follows:

Workflow Basic Information:
Name: Place any descriptive name
Record Type: Transactions
Sub Types: Vendor Bill
Release Status: Released

Initiation:
Event Based: On Update
Trigger Type: Before Record Load

Condition:
Status = Bill: Paid In Full

Click Save.
Go to Diagram > Click State 1.
Click New Action Button.

Set New Action settings as below:
Workflow Action: Remove Button
Trigger On: Before Record Load
Parameters: Credit