Here is the question - if you take [part of] payment in advance, spend it and still won't [manage to] accomplish the mission*? Will you be able to be in debt? What are the consequences? There should be some. Otherwise you shouldn't get paid before the task is done and not getting money would be punishment enough as you'll still have to buy other necessities (i.e.: spend money, or steal, or take a loan, etc) to get goods (health included) that'll allow you to carry on (on top of what you paid lost for preparing the action in the first place). I think ability to get some cold, hard cash before you get a reputation enough to be able to ask payment in advance is a must (or should be for those who aren't convincing enough) and opens interesting possibilities. I remember taking loans in Crusader Kings 2 or Europa Universalis 4 to pay off my long war or to start business as merchant in Space Rangers 2.
*It's pretty much the same as deducting money from payment, only better simpler, because payday happens when you're done with the mission and if you fail you don't get what was required to get paid, so there is no money to be deducted to pay for the cost of preparations, etc. and you're missing the rest of the payment, so you're basically "in debt". In this case it's not even deducting money from payment - as there is no payment - it's more like taking a loan you didn't want to take (as you wanted to accomplish the mission).