Use Project Custom Property for an Assertion value?

Yes you can. I always do it in my tests.
What kind of test step do you have problem with?

Just to be sure, you don't have ${#Project#SLA} as the actual project property? The property should just be SLA in the Project properties tab, and then expanded where you want to use it with ${#Project#SLA}.

Here are the steps to reporduce:

- Create a project custom property called SLA.
- Create a HTTP test step.
- Go to Assertions for the test step.
- Add a new one called SLA with value of 300.
- Edit the added SLA and replace its value 300 with ${#Project#SLA}.
- Click on the SLA again and you will see that the value is set incorectly to 300.

Ah, yes that seems to be a bug...

Here's a workaround. Put this code in the Setup script of the test case or a Groovy script step before the request:

testRunner.testCase.getTestStepByName('HTTP Test Request').getAssertionByName('Response SLA').setSLA(context.expand('${#Project#SLA}'))