Create a petty cash account.
When you are giving him a petty cash of 500
Transfer money from your main cash account into the created petty cash.
When he returns, open the petty cash and spend money from it entering all the expenses he has incurred.
Now if say the balance remaining is 100, do a transfer again, this time transferring 400 to that petty cash account to bring it to 500.
Now if he over spent and has claims, you can still enter all expenses in the petty cash which will give a negative balance. Eg -100 .
In this case transfer 600 from your main cash into the petty cash account for him, he will pocket 100 and use the remainder 500 as the petty cash new balance.
There are many ways to go around it.
You can also use the expense claim tab to record his expense claim and pay that one out differently.
I hope this makes sense