Why is it strange that he pays you before the auction? He's appraising the slave, estimating the minimum auction value and then building in a comfortable profit margin for himself.
On average, selling slaves from the Fogs to Farid is still profitable. Slaves obtained from the easier Fogs battles are less valuable, on average, than slaves obtained from harder battles. Using healing balm instead of the medical center improves the profit margin as you said.
You're playing on the hardest difficulty, right? On normal and easy, rental costs are lower, the disobedient phase is shorter, obedient slaves are minimally diligent regardless of their mood, and races are easier. Easy custom games also give you access to the in-game cheat menu, so you can do whatever you want without any challenge whatsoever.
I'm sure Master Vong appreciates your business. But are you doing this for supplemental income or just to survive?
To get ahead, you can train a D- or D+ slave with several specializations (to appeal to multiple buyers), give her a spa makeover and some stylish clothes, and sell her at auction. Or just train a few D+ slaves for district residents. Direct sales are more profitable in the short term than guild contracts.
In the long term, raising guild reputation lets you earn more sparks by sending multiple slaves to the arena every decade. But you can raise guild reputation by selling high rank slaves at auction, so you don't need to do contracts unless you're very short on sparks (prepayment and a free slave).
If you are doing a contract, consider training her to as high a rating as you can instead of just fulfilling the requested rating. As long as you can increase her value by more than 10 sparks per day, you're coming out ahead. If she becomes an arena champion, she can earn more for you each decade than you lose in late penalty.
