Does the client have to prepay ?
There is no obligation for your clients to prepay missions on Malt, so if you don't mind getting paid at the end of missions, that is absolutely possible, you can start working when the missions status is "mission accepted, waiting for provision".
This can be the case for example when you know and trust your client, or for bigger companies.
Indeed, most large companies have 30 or 45 day payment policies for suppliers for example. If you have any doubts however, do wait until the client has prepaid to begin your mission!