is there a way change the approval status of an approved invoice to rejected?

with right permission status can be flipped, unless there is a approval workflow involved. If workflow is involved you have detach the workflow first then flip the status

message has been deleted

If approval routing is enabled for Invoice(Approval Status field can be exposed in the form), maybe you can set it to Rejected as Admin role. Otherwise it might be possible via workflow. Why not just apply a credit memo to it?

Setting it to rejected manually or via workflow will have the same error notice.

I see. Might be a hard restriction then. What about adding a Reject button in the Approved State then creating another state(Rejected state. Set field value of Approval Status=Rejected) upon clicking that button?

Setting it to rejected manually or via workflow will have the same error notice.

I thought you said setting it manually would result to the same error?

Setting it to rejected manually or via workflow will have the same error notice.

Dig into where that error is coming from. I recently faced something similar and it turned out that there was a script guarding against the change which I needed to temporarily turn off. Via Customization >> Scripting >> Scripted Records you can inspect all scripts deployed to the record type you're facing issues with.

is there a way to show scripts in the record's system information? just like how workflows show in system information

No that I know of. But via scripted records you can see all deployed scripts and hopefully the name will tell you which one it is or you'd to eliminate by undeploying one by one