My manager asked me to enter his questionable expense claim and supplier invoice

In order to best protect yourself, you need to make sure that you are in full compliance with all policies in your workplace. Almost all companies do have an expense policy. Read yours and make sure you understand it. In almost all cases the policy will state which documentation and approval is required for specific amounts and expense types.

If the policy requires the documentation that your manager doesn't provide, then you should politely refuse. "Sorry boss, submitting this without a receipt is against company policy #xxx. If you still want to me to submit this I need a written exemption".

If what your boss does is in full compliance with the stated policy, than you are fine and you can keep doing what you are doing.

If there is no policy or the policy is vague and ambiguous you should ask for clarification from someone in finance. "I'm doing a some expense work at the moment and I have some problems understanding the policy on that. Could you pleas explain what the exact rules are we expect people to follow".

DO NOT sign an expense report on your boss's behalf. Most expense report systems require a signature with a sentence like "I confirm that these were necessary expense on the companies behalf and that they have been incurred and accurately reported". You cannot not confirm since you weren't there when it happened. That means you can't sign this. If a signature is required, have your boss do it.

Your risk is you will both be fired when accounting figures out what is happening

Expense reporting is a big deal and is an easy way to get fired. Recognize that if you submit false expenses you are both potentially stealing from the company and also falsifying records related to expenses and thus taxes.

If my boss asked me to submit an expense report without documentation I just wouldn't do it. "I'll put it in when you send me the receipt. I need my job too much to get fired for a bad expense report." If my boss got pissy about it I'd ask if we should both go see his or her boss about it.

A lot depends on where you are and what the laws of the country are. In some countries, you would be complicit of fraud.

To top that off, the manager could put all the blame on you.

You have a choice.

1. Refuse to do it and be prepared to find another job.

2. Carry on doing it but document it by email. For every transaction, send your manager an email, saying sonething like. " As per your instructions, I have entered this amount today. I would really like to enter a description for it please" And keep those emails, with the headers.

As everything and as the diverse answers here show, it depends.

If the context is a big or medium enterprise chances are that you are abused to cover a conflict of interest and most likely criminal behaviour. In other words saying this manager is stealing money from the company and abusing you to support it.

Unless it is a small enterprise and this manager is one of the owners of the enterprise. In that case it is just dull work entering half-correct bookings for the tax audit that might happen someday and reveal that receipts are actually appreciated.

A word of advise, if you in this moment realize that you might have been abused to execute a crime against the company, report this immediately. Do not do nothing as this can be held against you.

If you are concerned, it's not inappropriate to ask your manager for an explanation. If you are really concerned, you may want to ask your Manager's manager, who will hopefully keep it quiet while giving you direction.

Your manager is giving you clear instructions. The payments are properly approved. There isn't a problem here unless you make one. You're not signing off on them, you're just data entry.