### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Similar to https://github.com/BerriAI/litellm/issues/19781, internal users who have been previously set a budget cannot be set back to Unlimited.

<img width="2918" height="670" alt="Image" src="https://github.com/user-attachments/assets/467be4d4-47a4-4307-ab41-0c66668541b3" />

### Steps to Reproduce

Steps to Reproduce Precondition:

Max Budget is not set in Default User Settings Steps:

Create a new user. Its default budget should be "Unlimited" Update its Max Budget to any numeric value Save changes Update its Max Budget again to an empty value, signifying its budget should be unlimited Expected outcome:

The user's budget should be return to Unlimited Actual outcome:

A generic error pops up with the message "Failed to update user." The API response is the following:

### Relevant log output

```shell { "message": "Authentication Error, Invalid duration format", "type": "auth_error", "param": "None", "code": "400" } ```

### What part of LiteLLM is this about?

UI Dashboard

### What LiteLLM version are you on ?

v1.88.0

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