prepaid expenses to set against expense claim

Hi,
I just wondering the best practice if a employee was given say £1000 before trip which was recorderd as prepaid in month of payment and when employee return after 1 month , our company offset that £1000 againt their expense claim over the trip and pay remaining or employee pays back if expenses are less.
(i am New to company) 

what would be best enteries as employee expense claims are record under AP ( Account payable) ?

Any input will be appreciated.