What is this message when somebody wants to send gift money with my item?
It's someone trying to scam you.
There is no way to send Steam Wallet credit in the trade system. Anyone suggesting otherwise is looking to take advantage of you.
It is a common form of scam used to trick you into believing that you will get the steam wallet after you give the item to the scammer.
REMEMBER, you CAN'T transfer steam wallet funds. The only way to do so is to give someone a steam wallet code.
Please do not accept the trade offer, you will not receive the wallet fund.
More information here: http://steamcommunity.com/sharedfiles/filedetails/?id=232687419