I have learned the hard way to never pay developers up front again. However, developers have learned that they need to get paid up front. So this is a catch22 that makes it really hard to hire new developers on xenforo.com
As it stands xenforo.com is just not a safe place for this.
I would suggest to XenForo LTD to consider to add a cheap Escrow service to the paid development section. For example trough the API of Armor payments / Payoneer which is especially for marketplaces and handles not only the payments, but also the disputes. As the fee is really low, this would be a great enhancement to XF which could resolve a major handicap.